It certainly is not reasonable for him to pay only a breeding fee. Offer him first chance to obtain the foal once it's born, or offer to help him pick out a foal. She is ultimately your mare, and every breeding takes a toll on her body. If you want to keep another foal out of her, then don't allow someone else to breed here.
In a lease-mare situation, the leaser normally pays all expenses incurred in addition to a leasing fee. This includes vet, feed, hay, farrier, stud fee, insurance, and whatever else the mare or foal may require. Advise him that breeding is expensive, and that there are many other ways to obtain a foal for a low cost.
I personally would not do it. If he wants to lease her, advise him that he is responsible for all things involved. Sounds like he's trying to scam you to be honest. The "breeding fee" would be minuscule in comparison to the cost of the breeding itself.