Ah little grasshopper, that is an interesting question, and the answer is twofold:
1. Being jealous over something like that shows that you care for your significant other and for your relationship. Let him know that you are uneasy with the situation.
2. That said, you also must trust your significant other. Without trust, your relationship is without foundation.
I had a similar situation in august. A childhood friend of my girlfriends came to Finland for about 2-3 weeks (he's from Poland). And he stayed at her place during that time (she lives with her parents so it's not as bad as it sounds). Normally I would go to my girlfriends place for the weekend, but it wasn't possible now, since there was no room. So I saw her only rarely during that time. And during that time, she spent alot of time with him. I trusted her, but it did make me uneasy. One evening I called her and told her that I trust her 100%, but I'm still extremely unhappy with the situation, and I feel like I have been pushed aside. She listened to me and said "I'm really happy you told me that. To be honest, I would have been disappointed if you had just said "No worries, I trust you 100%".