Gosh, that’s a tough one Jo. I don’t have that situation myself, so not sure what I can helpfully advise, but I have a few ideas.
Your husband says quite rightly that he’s an adult and can make his own decisions, but maybe you could steer the conversation towards what he bases his decisions on? Would he watch Dr Malhotra’s presentation on sugar, for example? Gillian has provided a link to it. It’s quite powerful, and he talks about the ‘obesogenic environment’ we live in, which as you say most people accept as normal. Maybe your son would be interested in some of the things Dr Malhotra says about ‘big food’ – the big multinationals who are feeding the nation with what we all know is very damaging and unhealthy, just as the tobacco industry did.
You don’t sound controlling to me at all, just concerned for the health of your husband and son, which is reasonable.
I’ll be interested to know what other ideas people have.