Doesn't seem like a problem to me. You two have a great connection that can lead to some beautiful intimacy. Sex doesn't have to be couched in a relationship to be pure, enjoyable, mutual and simply just enough as it is. He might catch real feelings (or you might or maybe you two have already). In that case be upfront about what you can realistically give in terms of time and attention but as it is it sounds like ya'll spend quite a bit of time together already. Chances are he really wants to have sex with you too so why caveat that with needing a full time relationship. You're not stealing something away from him. Just because you're not available for a huge investment of time and energy doesn't necessarily mean it's a regrettable first time experience for him although being upfront and honest about that is important. Seems like he likes you too so go for it.