A free lease, with an option to buy, like
said, is what I did when I got Teddy. I told his owner I would free lease him for three months (payment for board up front) and at the end of that time if I thought I could do something with him, I'd take him. If not, I'd give him back. It worked out well for me. I do like the idea of having the current owner have the vet out first. A month into my free lease of Teddy, the owner was like, "He needs his vaccinations and teeth floated, and oh yeah maybe you can have the vet figure out why he won't take a bit." She hadn't been doing anything with him, so he was actually pretty overdue for all of that stuff, and the farrier.
But if you're thinking about him for your child, then for sure even before that I'd try a couple of test rides.
ETA, two more thoughts.
1. Your mare is pregnant. Can you afford FIVE horses once she foals?
2. Depending on the way the pastures are set up where you board, can you ask to have this guy put in with your other horses to see how they all get along? In terms of herd dynamics, my Teddy was the best thing that could have happened to me. But if could easily have gone the other way, too.