cabbagedad you can't say that someone who doesn't throw hard cannot make it but the likelihood of it happening is pretty slim. There are many guys out there at MLB, MiLB, DI and down who don't throw hard but get guys out but it's really not what the next level (whatever that may be) is looking for.
Let me explain it a couple of ways - first you can't teach / coach / create tremendous arm strength. Yes you can work your rear end off and make what you have better but you're not going to take a guy who has worked at it and is throwing 78 and change / increase his work and turn them into 90 plus. You can increase arm strength but what you look for is the guys who can naturally do it because the potential to work at it gives them a higher ceiling. The next level feels that they can teach a guy how to pitch as a softer thrower. They can be taught how to spot pitches, mix speeds and be mentally tough. But the thing is if they aren't able to teach that then no harm no foul - you really didn't lose out on a pitching prospect. You now have a second baseman or outfielder or whatever position.
Another way of looking at it is a guy who throws harder has more room to have an off day and still be effective. We got a kid who legitimately throws in the 90s and the rest of our staff is low 80's. We're pretty good team with the other guys on the hill but we're almost unbeatable with the 90's guy pitching. When our stud in on the mound he can throw three pitches out of the zone and still get a strike out whereas the other guys will probably walk the better hitters. It's not to say our 90s guy can't be hit because we lost the state championship with him on the mound and he pitched well. That's baseball but we actually had to make a nice comeback because the other team hit our other pitchers and got the lead.
In terms of getting noticed if scouts see a guy who can hump it up there he has their attention after one outing. A softer thrower can still impress scouts but it's going to take several outings to create interest. They want consistentcy from the pitcher. If the guy can throw it hard then they are going to take a chance with him and hope to teach him the other aspects - control, secondary pitches, mental toughness, holding runners. If they see a soft thrower have a nice performance they have to figure out if that's what he can do all the time or if he got lucky / the other team stunk.
Hope that helps some - it's not impossible for a softer throwing pitcher to get noticed and be successful but it will take a lot more work to get it done.
Tell your son if he wants to pitch at the next level then to go for it. Work his butt off getting a stronger arm but don't let up on the "other stuff" because chances are he won't turn into a guy throwing 90 (I really hope I'm wrong) but he can still be an effective pitcher.