Putting the collar on over the head can be done safely by turning it inside out. I have 3 HS prongs, all different. I also have a Lola prong which I put on inside out.I personally wouldn't put any prong collar on over the head for any reason.
(Most) prong collars are designed to be adjusted by adding in links or removing them so it's a case of matching up the collars gauge to your preference. A 3.0mm or 3.2mm collar will (usually) be 22-23 inches as standard with 1.25 inch links so you can remove a few to fit and add them back in as you need them. At 36kg he's may still be able to walk on a 2.25mm gauge collar with extra links added in but at that weight I'd favour stainless steel for the additional strength. If you aim for a collar at least 2-3 inches longer than his neck measurement that should give you the option to remove a link which can be added back in as he grows and you'll better be able to judge the final size needed before buying more links.