@asbxlhbrlsrw
Ok, just to make sure that I am understanding you. He can't get engaged to the girl in university because he was engaged via the phone call to the "other" girl. Are you saying that he can't get engaged to the "other" girl as well? The "other" girl is the one I meant for you to try building a relationship with, not the one in university. If the game is telling you he can't get engaged neither of the girls because he is already engaged, then there are still a couple of things you can try.
You can move all 3 sims into a household together temporarily. Use the CAS.fulleditmode cheat. Go to CAS and see if you can change their relationship status so that they are all roommates. Go back to live mode and see if the get engaged option comes back. If it does, then you're good, if not, then there's something else you can do but it will wipe out every relationship he has.
You can save him to your library and then delete the original and restore him to the game from your library. It's not the most ideal thing but it will clear all his relationships and then you can rebuild them.
Hope this helps.