As you've noticed, not everyone is good at getting along with new people. I bet his mother is probably treating you about like she does her son/other kids. I wouldn't take it "personally" but just use it as a learning experience. I remember my mother always made my friends go in the refrigerator and get whatever they wanted to eat and drink and could invite them, last minute, to stay for dinner; but when I went to their houses, even they weren't allowed in their own refrigerators ;-) and if they wanted me to come for dinner, they had to invite me many days ahead and dinner was the "exact" amount for each person, never very much or any leftovers and what they ate was different from what we ate, etc.

See if you can discover something she likes and work with that. My father-in-law had a weakness for chocolate like my husband and I'd make them both brownies when we'd go visit :-) Talk to her about her son, what was he like as a little boy, ask her advice on a few simple things relative to school, careers, etc. Find out her favorite TV shows and just kind of get to know her a bit if you can look beyond her rude behavior that is probably a bit of fear at this "strange young woman" in her home.

Does your boyfriend have a sister and/or you a brother? Could be your mom is more comfortable with your boyfriend because she has brothers, sons, more children, etc. and his mother doesn't have sisters, daughters. . .