The first thing to note is that your position with regards to this has not changed. As he is still with the company he still has the contacts. Thus if this seemed ok when you were employed then it is still ok. The relationship has not changed.
However I do note that there is some hesitancy in your part to the signing. I'm not too sure about this 50/50 split thing. Sure the contacts he can give you might get the company started, and it is only a might at this point but they are of a limited usefulness as time passes.
Once you use those contacts you still have a great deal of work ahead of you. And in all likelihood you will need to make other contacts yourself. But he will still get 50% of the revenue.
Points to remember
1) even if those contacts turn out to be completely useless he has 50% of the revenue from here to eternity
2) even if the contacts are good you still have a great deal of work and sacrifice ahead of you, neither of which he will undertake, but he still gets 50% of the revenue for doing nothing
I wouldn't trust this person unless he can show what he can bring to the table is more than his address book.