Try Edge brand shaving gel. They make several varieties, my favorite while learning to shave with a blade was the variant that is meant to provide extra protection against nicks and cuts (dull red container) and there are other variants such as for sensitive skin, with aloe, for tough beards, etc. I’d also suggest taking him to an actual barber (i.e. not a hairdresser, Supercuts, etc.) for his next few haircuts and have him ask for some pointers on shaving. Never tried this myself, but I know several middle aged and beyond adult men who have asked, even after shaving every morning for decades without problems, and all have said that it was very helpful.
I was somewhat prone to acne as a teenager and I found that using my dad’s electric razor made things worse - electric razors can have a somewhat abrasive effect on the skin of the neck, especially if your inexperienced at using one. This can irritate the skin and cause a rash, which often turned into acne. However, this was before wet/dry, lubricated, etc., electric razors were available.
Another bit of wisdom I can pass on from my teenage years is that the conventional wisdom of how it is best to shave (with a blade) after showering does not apply so well to acne prone teenagers, unless you wash your face very thoroughly afterwards. Shaving in the shower is probably optimal, but I’d recommend that he waits until he has enough experience that he doesn’t need to pay too close attention to what he is doing first. I’ve also found that minor nicks and cuts seem to take care of themselves during the duration of the shower if you shave beforehand. For this reason I still do so occasionally.
Finally, while multiple blade razor cartridges are great, they can often result in multiple cuts, i.e. if you slip and cut yourself really good with a Gilette Mach3 razor, you might end up with three slash marks on your face.