Count your calories - take 2 weeks, and count every calorie from any and all drinks, liquid and solid food put into your mouth - also add up the total protein, carbs and fat for each day.
The first week you will be on best behaviour - so that is why 2 weeks is needed, the second week is almost always a more accurate snapshot of weekly intake.
Once you have that work out what you need to eat.. the simplist way which i will go over here is to;
(1) Make sure your Macronutrient breakdown is around 30:50:20 - 35:55:10 - 35:50:15 (PRO:CHO:FAT). As someone who is naturally slim and lean (ectomorph) the carbs are very necessary - dont worry about simple carbs too much either, insulin is your buddy.
(2) Add 500Kcals to the total intake and add another 500 2 weeks after that. In 4 weeks see if you have gained, if no or 1-2lbs, add another 10% total caloric intake and keep following this every 4 weeks until you feel you are gaining too much fat, if you gained fat cut the calories added by half (250kcals) and continue along with that amount for another 2-4 weeks before adding more.
The reason the time-frames are vague is simply because you need to judge your progress on a weekly scale. Try not to look at your physique or weigh yourself inbetween, and when you do assess yourself do it at the same time (on waking is good) on the same day of the week.
This is to try to get a little 'control' into the assessment - tests are supposed to be controlled environments (not so much in this case).
Use the mirror to assess yourself as well as the scales, if you have someone to test your bodyfat with callipers, even better - but IMO photos and/or the mirror are the best bet - with photos being the most accurate representation of what you look like to others.
This is a very rudamentary way to improve a gaining diet - another 'quick fix' way is to simply take your bodyweight, multiply it by 20 and aim for that many calories in the ratios given above of pro:cho:fat. This is quite effective too.
I would normally totally assess a clients caloric requirements more thoroughly and in much more detail but as someone rightly suggested, this is not the place.
I think you have enough to go on there for 6 months.
For a first cycle, above 350mg/wk is plenty - with many opting for 500mg/wk (the 'if you going to get suppressed anyway, may as well do it properly' philosophy).
1000mg/wk is a dose reserved ONLY for those who are many lbs over their natural peak, those who have many years of AAS use under their belt. It doesnt take too long to get to that dose but it isnt a dose you'd take to start - ever. Not even you, with excellent genetics or very poor ones (incidentally, dont bitch about your genes, you have gained 60lbs of muscle in a few years and are tall too, you dont have it too tough believe me).
A cycle of 8 weeks of 500mg Test, shot @ 250mg 2x/wk with the first shot being 850mg to 'frontload' the dose, stacked with 30-40mg dianabol over weeks 4-10 would be a very effective cycle, able to give upto ~20lbs on cycle and likely ~10lbs retained post cycle.
This is a good achievement and a much more realistic expectation than 30-40lbs, which is possible for some with massive levels of fat and water from the lack of proper ancillary drug use (AI's).
You may want more water and fat - but i want to look like a low fat, well proportioned bodybuilder, not a bloated, menstrating woman.
You mentioned that you were getting ill at least twice a year, and these cold's/flu etc were leading to loss of large amounts of muscle mass.
It is NOT normal and healthy to become this ill, this frequently - especially for someone who does not drink or smoke (or do drugs i assume).
I drink much less than the recommended weekly allowance, and do not smoke either - and even i dont get this ill this frequently. When i get a cold i shake it off in a day or two max, when others are still coughing and spluttering 2 weeks later. This should be the case for a healthy bodybuilder IMO.
I suspect you may be overtraining - do you lose motivation in the gym in the week(s) leading upto the cold? Has your strength and size plateaued?
The only reasons i suspect this is due to the loss of mass - indicating a highly catabolic environment. Cold or not, you shouldn't lose so much muscle - unless it was an exaggeration?
As an ectomorph you are very inefficient at energy consumption - meaning you burn many more calories than another bodytype would, just to survive.. there are a number of theories for this, which aren't important for now.
However this means you would do well to stimulate muscle growth by training, with the least amount of energy used as possible. Look into HIT training or Doggcrapp - both very similar principles, and both do work for training periodisation meso-cycles.
It is based around 2-4 days training, for 45-60mins, high intensity, low volume - to reserve calories, allowing you to stimulate protein synthesis while preserving the calories ingested.
Look them up.
The extra food you will be eating will also assist in overtraining, and make sure you get in your 10-20% EFA's/DHA etc.. fats are essential in your diet, and are calorie dense.
Hope this helps.