My 15 dd will do that because of medical stuff. It is vasovagal syncope. They aren't "true convulsions" although they look like it!
A true convulsion is the inability to stay awake after the episode. How is he doing?
My dd has had this for a long while and she uses "visualization therapy" to get her through stuff.
In addition blood draws need to be done lying down from now on and make sure you have a cup of water handy and to
go slow and tell your DS to BREATHE.
Even so it still happens to her esp. when the attending person is not listening. She has learned to be direct with them and TELL them what she needs. Most of them are compassionate and comply however sometimes you get the person that says..."oh you'll be fine"....well, no she won't.
Last time she passed out was in the chair at the oral surgeon's getting the heart nodes on. Yea, that med person got the scare of her life.