I know you came to share your joy, and I wasn't going to burst your bubble, but the my beliefs got the better of me, and yes, that is what they are, MY beliefs, others mileage may very.
The age of starting a horse varies greatly, but to me the little guy looks a bit behind the time line, so I would choose to leave him a little longer before even sitting on him.
When I decided to have someone on him, then personally I would choose someone just a little smaller, and I am talking as much of a height thing, as a weight thing. Youngsters have trouble balancing, and the higher a person is above the horse, the harder it is to balance when a jiggle happens. Having said that I wouldn't sit a six year old on a baby either.
Last, I would much prefer to see a helmet being worn when sitting on a youngster for the first few rides, just in case something happens.
Good luck with your colt, hope it all goes as well as the first ride did.