If that is all the porn he watches then yes that is a red flag , but watching porn in general is not a big deal .
The way he responded to you was , not very nice and he could of said it in a better way .
( No one likes to be told what they can and can not watch )

If watching porn upsets you then you should make it clear when you date a guy that you do not except that . That way they know where you stand on that topic .

If you'r sleeping over at his mom's house then you should respect her wishes and not sleep in the same bedroom that is not odd at all , even tho he is an adult it is her house
and both of you should respect that .

I don't know how old you are but if you have your own place then maybe it's better to have him stay at your place for over night and not at his mom's place .

As for him calling his mom when he sleeps over at your place dose he have to or dose he want to big difference .

What I am trying to say is the only thing that I see as a red flag is that he is only watching porn that have a older woman .

Mom dressing sexy not a big deal that is just her style
It just seems to me you'r a bit hurt that he is closer to her then you .

Be honest with yourself do you feel a bit jealous ?
It's normal to feel that , I am sure his mom is and you'r feeling that vibe when your b/f shows you affection in front of her .

I don't know just follow your heart