I would like to think that if you have been with someone enough to consider them a "significant other", they should know what your schedule is and would know that you have only one free night that particular week. In a perfect world, an SO would honor that and want to spend time with you, but since this is a not-so-perfect world, I see nothing wrong with playfully suggesting that "Hey, honey, things are nuts at work this week and since Thursday is my only free night, what do you say to getting takeout and vegging on the couch all night?" Or something to that effect.
Basically, if you want to spend time with your SO, tell him so. Don't make him play mind-reader!
As for the going for drinks with the ex-girlfriend, that would make me rather uncomfortable, too, especially if I had been in that particular relationship for a length of time. Again, don't expect your SO to figure it out on his own, let him know, nonconfrontationally, that it makes you feel uncomfortable. If this is a long-term relationship and he still decides to go, I think it would raise a few red flags for me, but only you can decide what you can and cannot tolerate in your relationship.