Have you recently started ?
How serious are you with your training?
Do you plan to fight?
Also: How old are you? Big difference if you're an ambitious 16 year old or an out of shape 35 year old weekend warrior.
First of all, your first priority is being able to make the most out of the training. If you go to the boxing gym four times a week for 2 to 2,5 half hours, you might need 1-2 months to adjust. Aside from a bit o'cardio, like swimming, running or ropeskipping, weight lifting will be more of a burden in that period.
If you just go once or twice for an hour of punishing the sandbags and a bit of sweating, you're pretty much free to do what you want, as long as you take care of your shoulders.
Whatever your background, if you are really interested in learning the right technique from the start, you're probably better off to ditch the heavy ass weight.
While most myths around being muscle-bound are retarded, there is something to it.
Everyone who ever taught boxing technique to various folks will know what I mean.
For that reason, bodyweight exercises are prefered by most coaches.
That doesn't mean weights don't have their advantage, quite the contrary. But I'd wait until you have a good basic understanding on straight and hooked punches and how to apply them along with solid footwork.
Depending on talent this might be the case in 3-6 months if you're dilligent in your effort, but it might take as well much longer.
You could compare that to sprinting. Nobody will argue that a squat based strength program can have a significant effect on performance, but your technique has to be there first. Good sprinters usually do it in off season blocks and take their time to carefully condition their newly forged muscle.
So if your technique is good and your training efficiency is high, you can think about going beyond bodyweight, in my opinion.
Ross Enamait is your man here. There is much more then just bench pressing (in itself not a very good exercise for a boxer)
One more thing: If you're skinny and want to get a bit bigger, things get a bit more complicated. Boxing won't give you much in that department, as you probably know. In fact, chances are, you'll get skinnier.
A lot of middle weights and below can get a bit frustrated when they see their chiseled but thin chest and they assume bodybuilding will somehow give them full pecs and more power. This is tricky. Perhaps your good friend at the gym is a heavyweight and after adding benchpressing and curling twice a week on top of training, he looks and punches stronger then ever. But everyone is different, and most guys underestimate how much more they should eat to be able to box and gain (luxury) muscle (apart from shoulder, arm and some back which are the tools of the trade).
So if your question means in reality "I wanted to get tough and learn how to box but now I look even skinnier" then you've come to the right place. Gaining weight can be done very efficiently using weights.
Just establish a technique-foundation as I wrote before and slowly add mass buidling compound exercises. Then cut the boxing back and shoot for your desired weight.
Decide upon a realistic goal.
If need be do two mass cycles instead of one. Take your time.
Totally forget about bulking. You need quality muscle.
Classic exercises like the squat and the deadlift are great, because they don't interfere much with your training capacity and stack nicely with your usual boxing workout.