Okay it sound like your son likes the house and he'll probably buy even the buyers say no, no and no.
If this is true, don't tell anyone (even your own agent) but be prepared to haggle. Ask for the roof to be replaced and vent repaired. Sellers may come back with a counter-offer, which is normal, that they'll do some and not others. Now you decide if this is what you want, even through it may not be what you have asked for: it might very well be what you have expected.
In theory anytime you count-offer you essentially reset the table, i.e., the buyers can refuse and walk away. You cannot go back to the contract if they do. Similarly you can walk away if you don't like their offer. However so much into a sale I don't think the buyers will just walk away. See who blinks first and make your own judgment.
Remember there is no written-rule about how to negotiate. Ask more and expect less you'll be just fine.
