
How about suggesting your Dad puts all the leftovers in a dish so that they can be added to their evening meal? this way you can monitor what and the amount they have.
Or alternatively change them to being fed in the morning, so that his treats can be shared out then, and you can still make the adjustments.
My Dad is just the same when he comes round, thankfully not to often.
He looked after my oldest two a few years back and it took me six months to get off the weight they put on itn two weeks.