I certainly sympathize and agree with your assessment of doctors. They are great at diagnosing disease and illness (most times), but they are USELESS when it comes to getting anything done without medication, or with giving specific well-informed parameters for accomplishing things.
Here are some general guidelines. Everything can be modified, but these are general guidelines that should be adhered to most of the time, no matter what exercise plan you work on. Everything you do needs to have 2 goals : fitness goals and health goals (muscle building and keeping diabetes controlled).
1) Keep carbs low or tightly controlled. Only you and your husband know his carbohydrate tolerance, therefore unless someone is coaching your husband in person you two are the only ones who can watch it.
Obviously, as you know diabetics have seriously impaired insulin sensitivity and therefore handle carbs very poorly and require very careful handling with them. Exercise improves carbohydrate handling and insulin sensitivity around the workout window, therefore the workout window is the best period of time to have carbohydrates in the diet, both for muscle gaining and for health. (aside from Breakfast, which is usually the best "normal" time of the day to have carbs). Therefore while keeping overall carbs in the day low, put most of the ones you do have 1) at breakfast and 2) around the workout period.
1a) Protein is good. More protein in your diet provides an improved insulin sensitivity profile and also more muscle growth stimulus. So is fiber (though not around your workout period)
2) Quick movements improve insulin sensitivity and carb handling more than slow ones---This is reason #1 in favor of explosive lifting being the primary method of exercise, both in cardio (like sprints or bounding/jumping) and weightlifting. Of course, you need to get very used to general training form/technique and coordination FIRST--quick and/or explosive movements are only safe if done with coordination and focus, and a properly prepared body. But this should be a mid-longterm goal of exercise.
3) Whole body workouts or Upper/Lower split workouts achieve better insulin sensitivity and carb handling than body part splits (traditional bodybuilding splits). I know this will be a contentious issue for some people here on this forum, but for your husband's purpose it is very true. Whether it holds true in highly trained bodybuilders/powerlifters/athletes is something different altogether, and not relevant to this thread.
For newbies to weight training whole body each workout or alternating upper body/lower body splits work best. The rule of thumb is the more muscles you work at the same time, the more positive effect you obtain both in insulin sensitivity and growth stimulus for muscle gaining. This is ALSO the reason why compound exercises such as lunges, deadlifts, squats, step ups, overhead presses, barbell and dumbbell rows, and chin-ups are best muscle builders overall--they work many muscles in coordination and provide a bigger growth stimulus and fat loss stimulus.
The rule: the more of your body you use when performing 1 exercise, the better that exercise is for you in terms of health, strength, and muscle building.
4) Supplements---the most important supplements, aside from protein powder for your workouts, are health benefit supplements. Omega 3 fish oil does EVERYTHING. It's like the duct tape of supplements. Improves insulin sensitivity, cholesterol ratios, fat loss, digestion, is an anti-inflammatory, everything. Get the Flameout from Biotest, it's the best, period. Better than everything I've ever seen including the doctor recommended ones. If there were one supplement to splurge on, it's this one!
Big ones are omega 3 fish oils, multivitamin, vitamin D, Fenuplex and Insulinomics (both from Charles Poliquins website), and leucine. The big three for insulin sensitivity are the Fish oil, Fenuplex and Insulinomics.
5) Remember--as far as muscle gaining goes, while carbs can help they are not REQUIRED. Muscle gain has to do with training very hard to make your body adapt and grow, and eating over maintenance level to give your body the fuel required to grow more muscle during your rest days. You can do that with protein and healthy fats, no carbs required (although they can help a lot). The big thing is to watch the scale to see if you need to increase calories, and then keep a food log to monitor your calories so you know how much you are actually eating vs. what you think you are eating.