I think your son could clip into the old-style look pedals just fine. They seem to require about 40-lbs of pressure to engage with the tension turned down. I've heard that the Keo series requires more pressure to engage, but I haven't tried them and neither has my son. Likewise with Shimano SPD-SL.

My son was 65 lbs when he started off with the looks (they're model 5.1, but I don't think it matters). I turned the tension all the way down, and no problems. Just be sure to use the look-brand cleats, not the generic ones. My son had trouble clipping in with generic look cleats.

Look now makes a pedal that is supposed to have easy engagement - I think it's called the Keo Easy. I haven't tried it and I've never seen one.

Edit: Many will tell you that kids shouldn't be using clipless pedals. In my experience, they adapt just as quickly as adults. However, it's a good idea to start them out with platforms until they want to go clipless. This will minimize the amount of new things they have to get used to at once. I think it's a bad idea to start them out with a new bike AND new pedals at the same time.