Check with an accountant. I'm not an accountant, so I can't give you accounting advice.
However, aren't you over-complicating things? If you have the money to buy with cash, then just buy the home and sell it to him on a land contract. Why involve your IRA in it?
If you're trying to recapture $20,000, then do two notes, a first and a second. Make the first for $25,000 or whatever would be necessary for you to then re-sell the note to recoup $20,000 in cash. You'd have to wait maybe 6 months to season that $20,000 note, but that's one way to get the $20,000 back without involving your IRA.
Another way--if you've got enough money in your IRA--is to move your IRA to one of the self-directed administrators out there (if it's not already in one of them). You can then buy the investment property with cash from your IRA. Then you can sell it to your son--all within the IRA--on a land contract.
Bottom line is that it's difficult and often inadvisable to mix IRA and other money...and unsafe, too. You'd probably be better keeping it outside the IRA or all within it.
Again, check with an accountant for further details.