He could be allergic to DXM itself.
Otherwise there isn't anything in there to cause the vomiting IMO. Guafinesen and Sorbitol is what usually causes it.
Otherwise DXM itself induces nausea and some are allergic to it. Some can convert DXM-->DXO and DXO is what you want to get high on.
DXM causes the histamine release and dry heaves and such iirc.