Dash was giving him a warning. The fact that it didn't break the skin indicates that he has good bite inhibition...and just wanted to tell your son to stop.
At 6, your son should be able to understand if the dog gives you a warning, you need to stop what you are doing...not only with your own dog, but with other dogs as well. We are still working on this with my son, he is three. Maybe Dash's lesson will stick better than Mom's I never get carried away reprimanding the dog, if she growls at a child behaving badly (in this case also behaving normally). I do say "No"...in a low, firm voice, and give them a time out in the crate for awhile. If you blow up at the dog, you will just confirm in his head that there is reason to be uncomfortable when the child approaches.
Meanwhile you can work on strengthening their relationship by having your son do some basic training exercises involving lots of yummy treats. Have him practice calling the dog off of sleeping (or couch, or whatever) and reward him with something yummy. The dog will start to look forward to your son approaching.