Hey Popper,
Click over to Hodgdon's Youth Loads, they are under the Data tab on their site. Follow the reduction rule for the H-4895 with those 100gr bullets. Start him out light and work up.
I did that with my oldest now grandson when he was only 3.5yrs old shooting my Ruger Compact in .308. We started out with the minimum load, and 125gr Noslers. After shooting them 3-5 at a sitting mixed in with a 30 Carbine, or 22, he was up and running in no time with no flinch. After a few months of practicing, he dropped his first hog two weeks before his 4th birthday. I now have two more coming up, one at almost 5, and another at almost 3, that will both get to use those type loads.
On thing though, If you find he shoots the lighter loads good, depending on the velocity you end up at, you might want to go with something like the 100gr SST or 105gr A-Max, if it will stabilize for you. At a bit lower velocity these will work very well and not blow up on impact or slightly there after like they sometimes do at top end velocities. I switched over to the Barnes 130gr TTX for our actual hunting bullet after a 160yd broadside shot on a coyote failed to exit. THe close up shots worked OK with the BT but they simply did not penetrate on longer ones like the TTX did. I tested it out on over a dozen hogs at ranges out to 200yds and it worked fine on them. This was after I got the loads up to the 40gr range.
One thing about the H4895 loads is you can start with the Youth data, and work it all the way back up to the top end loads listed in the standard data. Thats quite a bit of spread, and I found that with everything being the same other than the charge weight, they still shot pretty decent all the way up to about a mid range standard load. I never got any higher with him and this rifle as he switched over to a 6.5x55 at 7, and then full loads in both the .243 and 25-06 last fall.
To be totally honest, I had a blast shooting some of the loads initially when I was testing them out on the hogs before he got a chance to use them. I wanted to be sure they were going to work, and trust me they did.
Hope this helps, and trust me I know all bout the long range stuff. My grandsons are up around Palestine and I am down in the H-town area.