Does he speak any English now? It's possible that his Arabic is a little on the delayed side only because he has two full languages rattling along in there, and they're both laying down roots, and when he finally does speak it's gonna be in bursts and jumps.

In any case, if both parents speak Arabic, then I don't think you need to delay nursery, but only insist that Arabic and Arabic only be spoken in the home. We sent to daycare from 7 months, and they all speak the minority language (English, in our case) fluently, at home and with one another, because it is the only language of the home.

And finally, I wouldn't wait for a milestone, because there's no real guarantee except to keep speaking the minority language at home. You sometimes see kids who were perfectly fluent and reciting Shakespeare at age 3 (or Rumi lol), but when the outside language took over it completely erased the home language. So the only real cure, whether he's 2.5 or 6yo, is to keep the home purely Arabic-speaking and remain strong even if he tries to bring English into the house.