Welcome!
I am an armature also but it has been my experience that. . .Expect it to take a while to get comfortable climbing.
Once you get on-rope, pull the leg pads down toward the backs of your knees to present a better sitting posture.
The belt & leg pads are interchangeable. Sometimes a large belt with medium leg pads (or vise versa) are the answer. I have a medium set of leg pads I can send you but what you probably need is a medium belt which I do not have. I have a complete small here which may work for your Son if he is a youth size.
A New Tribe saddle is not everyone’s answer to comfortable climbing. After you get some experience, you may very well switch to a Buckingham, Petzl, or any number of other _style_s.
Perhaps New Tribe can send you a medium belt to try with your large leg pads. Give em’ a call.
The Tree Climbing Basics video will help a lot. The best 20 bucks a new climber can spend. Also, all the training examples are done in a New Tribe saddle.
Click this _link_:
https://www.treeclimbing.com/Merchant2/merchant.mvcHope my 2 cents worth helps

Keep us posted!
Electrojake