If OH asks him to pick up something for the decorating on his way home he will often pick up the wrong thing because he can’t be bothered
Why do you assume he can’t be bothered? He clearly can be bothered if he is going to the effort of picking up, albeit the wrong, item. Perhaps he just made a mistake? We all do it.
OH was painting some doors yesterday after preparing them and when he left at 6pm said he will be back today to hopefully finish off before his kitchen is fitted in a weeks time. This morning my OH got a text message saying “don’t come over till 1pm” OH replied no, I’m want to start earlier to finish. Son replied someone is here, so OH replied Ok, have to leave it then. Another message came, ok come over at 11am.
Why couldn’t your OH have compromised with your son and went round at say, 12pm, given that your son, I’m assuming, had his girlfriend stay over? His starting time would only have been delayed by an hour, and the doors would have been finished for the kitchen being fitted, rather than your son being forced to agree to the 11am start time by threat of the work not being completed in time.
I think you both need to leave your son to get on with the decorating by himself and at his own pace. He is 45 and can manage this on his own. Him having depression does not make him incapable.