M.C.
Hi C.! I am sorry I couldnt help but giggle at that last remark about the shopping cart. My 4 almost 5!!! year old does that everyonce in a while. While she doesnt put the hands in her mouth constintly she still has her thumb in there once in a while.
I do have a question Does your son attend daycare.? If so this maybe were he picked it up or why he continues because the other kids do it. If not its just a phase he will give up in time, Till then just keep a bottle of Hand santatizer handy in your purse at all times and take a hand towel or tee towel with you to the grocery/whatever store that you know hes going to be riding in a cart in and seriously take string or yarn and tie the towel around the handle of the cart, It works as long as u knot the strings on the bottom so he cant get to them, I know it seems silly but if you are that concered about him chewing on the cart (and i dont blame you it is YUCKIE) then try it what can it hurt right?
Good luck and have a great day.