I imagine that if you felt it was easier just to leave him to his own devices, then you wouldn't be here asking GNs for ideas or advice. But, difficult as he is, it would seem that you have a couple of possibilities for dealing with this, as he refuses to confide in or be straight with you.
If you are registered at the same Surgery, then it should be simple to make an appointment for yourself and explain to the Doctor that you are extremely concerned and that you are there to GIVE confidential information regarding your Husband; that you don't expect any discussion or revelation from the Doctor regarding her patient, but that you feel very strongly that she may not have been given correct information, that your Husband is concealing his worsening health and possibly making whatever condition he has a great deal worse. It's you who are at the sharp end and who will be expected to pick up the pieces as his health worsens and that you have no idea as to what you can do now. That you just feel helpless.
If you don't want to confront the Doctor or explain your concerns in person, then you could write confidentially to the Doctor in question, giving a very clear resume of what little your H may have told you, what his daily life is like since his hospital admission, what sort of things he eats, whether he actually takes any medicines etc., Give clear details as to the worries that you have and state that you understand the Doctor's position and don't expect anything more than confirmation that this information will now be included with your Husband's patient notes.
However, it might be a very good idea to ask if it would be possible to have the surgery contact your Husband with an early appointment for a 'review' or 'tests', so that the Doctor has a better chance of deciding whether your Husband is likely to come to any harm if he continues to keep any health problems to himself. A diet sheet would surely be a very sensible idea from the surgery, so that 'the person in charge of meals' etc., can have the opportunity of providing appropriate foods.
It's possible that your Husband may simply have the idea that full fat milk could replace some of the weight he is losing, with no knowledge that drinking a lot of milk could be aggravating his health problems.
I would think that even though you may be unable to change your Husband's attitude towards your involvement in his declining health, the sort of action outlined here will at least give you the reassurance that you've alerted those better equipped to help him and done the best you can with a pretty impossible situation. Good luck and try not to worry.