Liberty Belle, ultimately, you need to follow your heart and head about what kind of rules and boundaries you want to set for your DS in your home. I think if you were really ok with it, you wouldn't be asking others' opinions

If you really are ok with it, go with it!
I have a DS16 and he does have a girlfriend. The rules we have in our home is that there are no girls for him or no boys for DD13 (thank goodness we don't have to address that yet really.) upstairs in our home. Whether it be a girl/boyfriend or "friend". Personally, we don't think it's appropriate, but more than that, for me, it's about boundaries. We welcome our kids' friends with open arms, and it seems like it's well received, as it seems like we are the hosting house where our kids' friends like to hang out, sleepover, etc.
They are welcome to hang out in our basement, family room, kitchen, etc. but upstairs in our bedrooms is off limits. It just feels to me that our home needs to have somewhere that's personal. Not that kids can't get into "trouble" anywhere else, it's just that its more intimate and personal in our bedrooms. For me, it's more than "trust", it's boundaries.
I hope your DS enjoys his friend's company today~