My recommendation for the 26er is indeed of agility and handling. Of course he will ride a 29er in the future, but the different between the future and now is his size and power. It's a little bit heavier to get the 29er on speed than a 26er. Imagine, you have work hard to get the bike rolling, 'hard' to handle, 'less' agility. All together, give him that much fun you want him to have ? I don't know. The important thing is having fun. A lot of fun. And fun is related to his skills. Technique, as I said before, is so important. With good basic skills, it's so more fun to ride. Think about loose sand, without the correct technique, it's no fun to ride trough loose sand. Even on a 29er.
Let him start on a 14" 26er. Let him work on his skills. And when he wants a 29er after a year 'hard' training (=playing) on his 26er, that shouldn't be a problem. There's a big market for 14" bikes. Here in The Netherlands is hard to get a descent 14" mountainbike.
As I said, my son now rides a 24er. It fits him perfect now. He have a lot of fun with the bike. He's learning a lot, he's working on his physical fitness, he can ride trails of 30 km without problems. When he's home, he's tired with a good feeling. It gives him so much fun. And I think he wouldn't have that if he had a bike that was hard to handle. His new 26er is now waiting for him. Soon, we will make a ride together, first on the road, second off road, third single tracks.
bottom line: FUN! It's all about fun!