Dragonboi, just tell him that you are scared. I'm sure he'll tell you there's nothing to worry about. Your boyfriend sounds pretty levelheaded to me. As I said previously, there's no way you can tie another person down, not if they are intent on following their desires, but you could come to a rule that if he "has to" stray, then he'll be careful and remember to use protection, for your sake, his sake and the other partner's sake.
After all if it's only about sex, it's not going to kill you, nor, probably, your relationship. I wonder how he would feel if you gave him that liberty. Would he consider it as you not caring? Would he, on the contrary, see it as a token of trust? You can ask how he would feel about that kind of arrangement. If he has that liberty, would you wish to have the same liberty? Make sure that he understands that YOU have no intention of renewing the cheating experience, if that's how you feel about it. Cut him that slack; he'll know you trust him, he'll do everything he can not to disappoint you. If you end up losing him, it's probably that you've both failed to renew your respective "engagements". But were there ever any engagements taken?
I would say that with his history of depression it's pretty healthy for him to have a few friends, including some gay ones. Think about it in that light.
Good luck, Dragonboi.
PS I asked whether you two lived together, but you didn't answer that question. Sharing the same living accomodation can make things difficult, if other intimacy is to take place, ie not intimacy between the two of you, but that again, is something you can discuss.