I'm sorry this is happening.
Does he have the sort of job where he goes on business trips? Just on the off chance that it's nothing. I used to be away from home 200 - 250 days/year (it stank!) and often left with guys but it was for auditing and nothing ever happened (and I didn't want it to, no one suggested it, etc.). Essentially my point is that it is entirely possible for people to have productive business trips with no subtext.
you have his coworker saying something. Could the coworker have an agenda, giving you this information? It's up to you to determine whether the coworker's information is reliable. Certainly the trip is real as your boyfriend did not deny it, the question is whether it is for anything beyond business.
I'd like to know why your boyfriend got angry. Because the coworker is a blabbermouth? Because you jumped to conclusions? Because he was caught? Because he feels its upsetting the fragile nature of a reconciliation relationship? Something else?
Whatever the answer is to that question, that's going to be the driver of things. So ask - calmly, not during a fight, not during other things. Explain what you told us here, that if it was something from when you two were not seeing each other, hey, it happens, but you still expect the trip to be cancelled. And if it is a business trip, then you believe that that's all it is (and you have really gotta say this, and give him the benefit of the doubt unless you have evidence to the contrary that is beyond a nebulous feeling). And if it is more than that, then you want to know what's going on in your relationship, and whether it's got a chance.
Best of luck to you.