It appears J can definitely hear. At least some of the time. We still don't know why he doesn't startle to loud sounds, but he can hear.
H was playing with him. H said "Hi". J immitated H perfectly, and said "Hi" back! I think that's a pretty good sign that our little boy is not deaf. Now we just need to find out why he doesn't react to certain sounds.
J also gave his first social giggle tonight during play time. It was precious!
Anyway, we still need to go for his hearing evaluation, but I am temporarily relieved.