Well, then you have the following choices:
1) You can get in with him while he's still single, the consequences be damned. The advantage? You get what you want. The disadvantage? It probably won't last, because you're not actually sure what you want, so that would be rushing, and hence inadvisable.
2) You can decide that he's not for you, and stay well back. You should probably tell him that you're not looking for a relationship with him, so there's no problems with mixed signals. Again, the advantages of this is that it allows you time to assess how you really feel, and hence make any move you make in the future more certain.
3) You do neither, and talk to him about how you feel. Advantages: you get the air between you cleared, and you both know where you stand with each other. He can tell you how he feels about the situation, and you can work out a decision on whether to be together or not. You
have to be clear and open and honest with him, though, otherwise the decision you make will be flawed. He has to be the same, too, but I gather you'd make sure of this, otherwise you wouldn't want to go out with him

. The disadvantages: if you don't reach an agreement straight away (and decisions like this are best when time is taken, I think) you leave the possibility of him going on to find someone else while you're trying to figure out how you feel.
I can't tell you which option to take, although I think taking your time when you feel indecisive and unsure is the best course of action. If your gut feeling says you're not sure, then stay away from relationships. I also think communication is important, so my advice is option #3. You may have other options, but from my outsider perspective, that's my read on the situation.
Hope it helps

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