You could say all the same things (roughly) about the girls in Toddlers & Tiaras. The harm is not about direct injury for participating (although see below*).

He's a kid. Little kids don't go in for weight lifting competitions on their own. I'm sure if you asked him he would say he is into it but face it. He is into it because dad is into it. I doubt it is different for the little girls in Toddlers & Tiaras.

Run around a playground and poll the little kids and see how many would answer that what they really want to be doing is weight lifting (or enter beauty pageants).

I am not opposed to weight lifting nor am I opposed to beauty pageants. It is putting little kids in those things that is the issue. When he's older, if he wants to lift, he can choose for himself and more power to him.

*Edit to add: Forgot --

His growth isn't going to be stunted, his back isn't going to break, though his confidence might inflate.

Glad to hear it doctor except...:

Strength training, not weightlifting

Don't confuse strength training with weightlifting, bodybuilding or powerlifting. These activities are largely driven by competition, with participants vying to lift heavier weights or build bigger muscles than those of other athletes. This can put too much strain on young muscles, tendons and areas of cartilage that haven't yet turned to bone (growth plates) — especially when proper technique is sacrificed in favor of lifting larger amounts of weight. (CITE)