Question about a recently retired parent.

A few months ago, my father retired from his career of 35 years. Since then, he has been acting most unusual, and we are getting concerned for his health. His level of activity has dropped to almost nothing. He sleeps until late in the day, never works out, has no hobbies, etc. Whenever my family, or anyone else, encourages him to get more active, physically or mentally, he just gets angry, and lashes out. We all acknowledge the sacrifices he has made for us over the years, and that he deserves some degree of a break from things- but surely retiring isn’t an excuse to simply shut down, is it? I try to help out with the chores, but I both work and go to school, and it simply isn’t fair to my still-working mother, who feels as though she isn’t getting the support she needs. He has gone to a doctor, and been prescribed a number of medicines, but it has gotten to the point where he is afraid to take more, and we worry that side effects might be part of what is causing his lethargy. Is it right for us to complain, or should we simply back off? Is there a way to convince my father to get active again, without evoking his anger and resentment?