The 25 WSSM is about the equivilent to the 25-06 so recoil shouldn't be that bad. To say it was too strong for your 9 year old would depend on the child. It is not strong recoil but what other calibers has he shot with?
His age is not really the determining factor of his ability to accept recoil. I've been a competitive skeet shooter for years and have wanted to teach my brothers kids the sport. Most professional instructors (see Fred Missildine's book) will advise you not to start a child until he/she weighs 75 pounds. For skeet shooting that would be the min weight using a 20 ga autoloader to start. To much recoil to early develops bad habits...... the body will naturally avoid the sensation of pain. The kid may develop a flinch or a hesitation to pulling the trigger or something like that, all bad in shooting sports.
I'd use the same guide lines for rifles.
Don't assume that when fall comes, or hunting season, that the big bucks, and big bulls leave. Whitetail deer bucks especially, simply hide. And they hide very well. Mule deer bucks may leave a little. Bull elk may leave, but don't assume they do. Don't assume any of them do. I mean - where would they leave to? (Just another place where, for the most part, they would be hunted.) These big animals have the ability to hide in very small, very obscure places. So, embrace the fact (...