If there is an offer to sell the car, then the seller can't change the terms; if he does, the buyer should be able to reject the deal. So, for example, if the seller is supposed to replace the shocks, has to do it; you would have the right walk away if he refuses to. In terms of how he'd have to do it, that depends on the agreement you and the seller had come to: if the agreement was they'd have it professionally done, then unless that happens, you should have grounds to rescind. However, if you had agreed that the son could himself do it, you'd have to let him try. Or if the agreement was silent as to exactly how it would be done, you could try to reject and if you end up in court, the court will hear testimony and determine what  the mutual understanding of the two parties was as to how the work would be done--and possible, if the court concludes that you reasonably believed the work would be done professionally while the seller reasonably believed his son could do the work, that might be grounds (lack of agreement) for  the court to void the contract.