Sounds as if you have already talked about it, pelafina, and got nowhere.
Obv, it has got something to do with him as the health of his son is presumably important to him, and he has a right to express a view on this...trouble is, his view is that there is no health impact and it doesn't matter. It also has got something to do with him because your bf relationship is important to you and he should take your well-being and your feelimgs seriously.
His discomfort is a feeling, too, of course, but it should not weigh as heavily as your own desire to fulfill your and your son's needs.
His discomfort can be dealt with - he's grown up, he knows there's nothing wrong with bf, and he knows all his son is doing is eating and drinking.
Your feelings if you were 'made' to stop would be less easy to overcome. Resentment, disappointment, sadness, regret, anger....they're a lot deeper than momentary embarassment.
Time to talk again??