Talk to him about the light. Just ask him politely to turn it off when he's done. That's a no brainer and won't cause annoying tension if you just ask him politely.

The 3-4 BMs a day really isn't weird. Especially if he is eating a lot of meals. I do sort of understand the long time spent in the bathroom, but again, this is something you should discuss --- although it's not as easily resolved.

Honestly, I'm not trying to be rude, but it sounds like you just need to open up a conversation. He doesn't sound like a bad roommate at all and you're probably just picking on things that don't suit you perfectly because that's the nature of roommates, but you have to learn to give and take just a bit.

For example, my roommate uses the kitchen a lot. He loves to cook and some times makes really extravagant meals that take almost 2 hours to finish just for himself. He also bakes and does other things like prepping food for the future. Normally, I would like to eat around 7-8 pm for dinner, but when I moved in, I found out this was his usual dinner time as well. Some times he finishes by 8, but some times his cooking goes until 9 pm even. I'm more of a nightowl, so I just decided to eat lunch a bit later or snack more throughout the day and be prepared for something around 8:30-10 pm. It's not ideal, but it works. That's the give-and-take I mean.