Please bear with me, this is going to sound like absolutely nothing but it has been rumbling for weeks now and I have no idea what to do, if anything.
DP's nephew has been living with us for about 18 months. He is an orphan who was brought up by DP's mother till she couldn't cope, which is where we stepped in.
I know that some people may remember our story. I don't want to spell it all out now, for privacy reasons, so will have to opt for drip feeding I'm afraid.
DN loves sweet things. I am very generous with puddings, with chocolatey things quite often, or ice cream. After what we can call a lengthy and at times painful training period 
he now always eats his dinner, including vegetables and salad or whatever, cooks at least once a week, dishes in machine, sets table and does not dare take food from fridge. We never have biscuits or crisps in the house and only yoghurt or fruit. He is also polite and works hard at school. I think it is little short of a miracle and I'm really proud of him, and yes, of myself too, for all we have achieved.
For a long time I had trouble getting him to have breakfast before school - and as he was refusing school when we got him, and behaving very very badly when he did go so (his behaviour nosedived when he was tired or hungry) bedtime and breakfast were big issues I dealt with.
Also, I didn't know him at all when he came to us and food, and cooking, has been the start of our bond.
Finally, after much negotiation and trying different things, including cooking him bacon and eggs every morning, porridge with syrup and fruit etc. we settled on chocolate weetabix. He loves them and gets through a box a week, which is quite a lot because he snacks on them too, after dinner before bed, at the weekend etc.
This is all me, rather than DP who is working very hard at the moment and frankly hasn't got very involved in any of it.
Suddenly, DP has butted in and said DN can't have chocolate weetabix, because it's baby food and ridiculous he has to alternate with normal weetabix. He hates that he has put on weight (he has a bit, but is still within the normal range). I asked him to do more things with him, some sport or whatever (we live by the sea with loads of stuff to do, it's the main reason we moved here) but he's too busy so just makes a change that I have to enforce and go through the whole nightmare of getting him to have breakfast, etc. again when I had it all sorted. I have talked to DN and he's really upset he can't have them, we had a look at the box and they only have 10g more per 100g than the normal ones which he'll pile sugar on anyway. I'm so pissed off with DP, have tried to talk it through but he seems to think it's The Way Forward and I mustn't undermine him. We have had about four massive rows about this and I no longer know what to think.