What is the percentage of damage now as compared with previously? Either offer that (say the damage now covers twice as much area, so offer half his estimate ($250)) or else require him to get an estimate from a repair shop you trust; they need to know the full story about both accidents. They will almost certainly be able to tell which damage is new versus old. In all your dealings with him, make sure to include a statement about the original damage your son caused being significantly less than the new damage that your family had no part in causing. A paper trail showing that there is extensive new damage is needed in case he takes you to small claims court to get you to pay the cost.
Also, you and DH need to get on the same page about this--find a compromise position you can both live with and then each of you need to abide by it--no going behind each others backs. If one of you thinks you need to alter your compromise, it needs to be discussed between you two before it ever gets ANY discussion with anyone else. For example, if friend makes an offer that falls outside the agreement between you and dh, dh needs to say, "we'll think about it and get back to you" not "well, that seems reasonable."
Also, you and DH need to get on the same page about this--find a compromise position you can both live with and then each of you need to abide by it--no going behind each others backs. If one of you thinks you need to alter your compromise, it needs to be discussed between you two before it ever gets ANY discussion with anyone else. For example, if friend makes an offer that falls outside the agreement between you and dh, dh needs to say, "we'll think about it and get back to you" not "well, that seems reasonable."