Just as a tip, you should allow him to text other girls, don't let that bother you.
But.... Texting some other girl 24/7 is a hint that something is fishy.

Although, what you said about the girl. How she is prego and a lot older, maybe it isn't sexual. Maybe he is just giving advice/cares about the well being of the co-workers. To explain why he freaked out on you. Maybe he's embarrassed about the situation.

Don't be Paranoid. Paranoia brings down a relationship. But you can be cautious

Ok well re-reading you're original post and this, you seem like you're getting fucked over by your Bf.

You need to deal with this head on, don't beat around the bush. If you don't set clear guidelines, and/or he's breaking those guidelines. You need to confront him about it.
The fact that he was asking your friend those things, is a huge red flag.

Personally, if my SO did all that stuff, it'll be near the end of our relationship

Oh god no. Never blame yourself for his actions, that can lead down to a horrible, horrible trail.
If you feel like he's going to do that or something like that again, end the relationship. Don't be in a relationship that you're being overwhelmed by the feeling that he's going to cheat. It's just going turn in to a breakup that is long overdue.

AHHH! You just said the secret word! MINDSET! Clint Eastwood once said "I don't believe in pessimism. If something doesn't come up the way you want, forge ahead If you think it's going to rain, it will"

In other words if you put yourself in a mindset where you think something bad will happen, it will. Optimism is the best thing a person can have, because not only does it change your inner self, it'll change physical outcomes around you.