What are the issues that make this different from a normal buy/sell agreement? Is your son going to get a conventional loan to buy the house or is he getting a loan from you?
If you are going to accept less money than the house is worth, the IRS will look at this as a gift from you to your son. The gift tax rules would then apply. A similar situation arises if you give him a loan (or he does some sort of payment plan instead of paying 100%). In that case, you would need to make sure the loan meets the IRS guidelines for a family loan so you can avoid the gift tax issue.
As far as the lease goes, the same idea applies. If you are going to pay rent which is normal for the area, you have no issue - just do a normal lease. But, if you are going to pay less, your son could run afoul of the IRS regarding a gift (same applies if you pay more rent, but you would be the gifting person). I have not researched this situation though, so I do not know how this is exactly handled -- I just know that you need to pay market rent to avoid it.
I am not sure how your state handles taxes to know if there is an issue.
As far as the realtor goes - I guess it all comes down to your written listing agreement. What are those terms and how long does it last? I've never signed a listing agreement that lasts more than a year, so I would think there is no formal problem regarding avoiding the seller's commission. If you feel bad about it, you might offer up lunch or something.