RST and Polywad make 2.5" shotshells, no need to stress the old gun with 2.75s.
http://www.rstshells.com/rst_classic...shotshells.htm
http://www.polywad.com/vintager.html
Buy a few boxes every birthday and by the time the kid has a hunting license he will have a long term supply.
I won't venture a guess on dollar value. Are you going to take it out of the boy's allowance?