Nitrous Oxide

My 9 year old son had some dental work done today.  He had 5 fillings on one side of his mouth.  They used Nitrous Oxide and said that he should be totally relaxed and do just fine.  3 of my other children have had nitrous oxide for fillings and did just fine.  However my son today vomited during the procedure.  Which they immediately extracted with their suction.  They said that this does happen some times.  In looking this over at information online, I have seen it say that this occurs when over sedation occurs.  My son remembers that he vomited, but what seems to bother him most is he said he was so dizzy and felt like he was spinning the whole time.  And that he was in a inclosed small space the whole time.  So I'm assuming he was claustrophobic.  He has to go back to get a few more fillings, and I'm wondering if he should not have the nitrous oxide?  Has anyone out there ever experienced vomiting during the procedure when having the laughing gas?  My other concern is if he doesn't have it, and he starts to gag or vomit that he'll freak out if he isn't lightly sedated.  I know from watching it, it was so scary to see him lying there vomiting, and not being able to do anything to help him, other than comforting him.  Any advice would greatly be appreciated.  Thank you!