20 g is no disadvantage at all. I would either look at the NEF 22 inch youth single shot or the youth offerings from Rem.
Another maybe better option would be to buy a 870 express with a 26bbl as they seem to pattern better than the short barrels and buy a knoxx adjustable stock so that it grows with him.
As for chokes and loads. There is no room to scrimp with a 20. I would buy him a Pure gold http://gameacc.net
in 570 which should provide 40 yd killing patterns while also providing even patterns and room for error.
As for loads the Environmetal 1 1/4 1250 6s duck loads are the best available right now and outpattern the turkey variety from EM.