I would be pissed if someone referred to me as a "little boy". Because I wouldn't call someone a little boy unless I thought they were under 12 years old. The implication is that you're less capable than someone who's older, and f that. I wouldn't have flipped out on him, but I would be just as pissed as you are.
When I first started T, I was often read as a 15-16 year old boy and the way people treat boys that age is ridiculous. They either don't think they're capable of anything, or they think they're doing something wrong. I had so many condescending remarks thrown at me in my first year of T, I can't even put them all here. I never reacted to it because I'm really passive and hate offending others, but honestly I wish I had. It's total BS when someone is treating you like you're inferior to them.
The good news is that it'll pass. Teenage boys have to put up with it, and trans guys have to deal with it when we start T. The key is to remember how it felt and be a better person when dealing with someone who looks/is young.