Can you teach him some coping methods such as slowing his breathing deliberately or having him breathe into a paper bag if he is hyperventilating? It sounds like he has some pretty extreme separation anxiety. There are some good books for kids with anxiety. One we've used is "What to Do When You Worry Too Much" and compares worrying to carrying for tomatoes that get out of control.
If you feel like your DS is having panic attacks and his doctor does think it matters, you might want to seek a second opinion or try a psychologist.
We are currently trying to figure out how we want to proceed with our DD's health care. She has anxiety and lately has been very depressed (she is only 7). It's been really hard, but our ND is on board. She wants us to get an evaluation in addition to counseling which we already just restarted.
Good luck to you.
I am a 40 year old unschooling, belly dancing, artist-mama of one almost 8 year old. I just had brain surgery and about it a bit because it's just so surreal.