OP, just taking it back to your original post, you said:
"... I won't take the key back of him. My DH feels very strongly about him having a key and wants it back saying he doesn't need it - thoughts anyone? It's causing massive problems between us, but I just can't ask for it back it feels so final! Is my DH or I the one being unreasonable?"
In this you are clearly wanting your son to still have a key. 9 pages later have you changed your mind on this? I know you said you'd email him to ask for it back, but this is a bit a half hearted solution because it gives him plenty of time to make himself a copy.
So, if you really want him to have a key still, and just feel browbeaten into attempting to take it back, then you need to do some serious work on your marriage. If you both want DS's key off him and the only blocker is that you are scared of DS's reaction, that is less worrying in terms of your marriage, and if I were your husband I'd appreciate knowing that.
By the way, taking the key back or changing the locks is not necessarily final. Some people do an awful lot of growing up in their 20s. Moving out might be the making of him, and in a few years he might be a different, much more responsible person. You might give him a key for him to catsit or something, and never bother to have it returned. But right now, your DH has made a reasonable request. Where house security is concerned, we tend to work on a failsafe system. If DH wanted all the windows locked and I wasn't bothered, we'd lock them to satisfy DH as the more nervous one. If I wanted us always to use the door chain after dark, DH would respect this and use it too, even if he thought I was being daft. How would you feel if your DH gave a door key to someone you'd learned not to trust, and refused to take it back? Would you be happy if he emailed for it back, knowing this would give time for that person to make a copy? Good luck with sorting it out.