I started my son with a Rossi single shot. It made a good trainer when he was not big enough to handle a pump. But I understand the problems with the hammer while hunting. I have not seen a hammerless single that I can recall.
If weight is the issue, most 20 ga pumps are not much heavier than a single shot. If you are concerned about safety and only want him to have 1 shot, only let him load 1. After he has the experience and you trust him with more he will already have the gun and be familiar with it.
I teach hunter education and I am the live fire instructor. We have 20 ga pump guns only. The average 10 year old ( both boys and girls ) handle them quite well. I have had a few 7 and 8 year old kids that did fairly well with them as well. With instruction and practice operating it he should be able to use one safely and effectively.
You can't fix stupid....however ignorance can be cured through education!