Run the net price calculator (take screenshots). If you can afford it, there is no harm in switching to ED. He can still keep his application in at #2 (he wouldn’t have to withdraw that until he hears positively from CMU, and it seems he will hear very quickly).
I would not take this offer as a good sign, nor as a bad sign. It is in the interests of the school to encourage you to do this. However, since your son was going to apply anyway, it is his first choice, and is an excellent school for his major, there seems to be little to lose. If the aid doesn’t match what you expect based on the NPC, your son will be able to back out of his agreement.