How to react?

My BF and I have recently gone vegan because I found out that I am allergic to eggs and dairy. My health has become an issue and I have had lots of problems because of it.. which is how I found out. My BF joined me to be supportive and he's hoping to drop a few lbs cause we both quit smoking and gained approx 20lbs ea.

Long story short, he is going through the withdrawals and feeling horrid yesterday so, I stopped by to check up on him and bring him some food. While we were sitting there talking, his mood improved greatly. He was smiling and laughing. Then his mother showed up with his son. I swear it was almost physical. I felt the atmosphere change. His mother, who is staying with him temporarily walked in with all the food stuffs that my BF is trying to give-up. Then his son walks up with a coke laughing and says to my BF, 'So, you're on some super weird diet thing huh?'

That is one example of how he is being undermined in his own house. He says, 'no soda.' She buys it for the kid and sticks her tongue out and laughs at him. Literally.

I made dinner for them once and made a regular salad as a side. That lil boy looked at me as if I had stood up and pooped on the table. I mentioned it to my BF and he doesn't know what to do. He tries to eat healthy, buy good foods etc, but he is undermined at every turn. Idk how he can live with the amount of disrespect he gets from all ends.

His own mother undermines him as much as his ex does. She will be moving back into her own house in a few months.. in the mean time I believe she is doing some serious damage.

We have been dating for 4 yrs now. One day we would like to move fwd in relationship, but I am also apprehensive because it also means I have to accept the mother. Lol

I am perhaps just as frustrated as he is. He loves and respects his mother. He's the most dedicated man to family that I've ever known and they keep shitting on him.

What would you do/say?