Any poo that may find its way into the foreskin tip will be washed out with his next urination since urine is sterile it is the perfect cleaning agent

At least for that part of the body

It is very possible that he will become not retractable again it goes thru stages and it isn't a big deal at all if it happens. As long as he can pee with no issues.
I love the idea of a spray bottle of water for really nasty poos wish I had thought of it

The foreskin is designed to keep stuff out the tip is a sphincter just like the anus. It only relaxes when they need to pee so nothing is going in there very far. As they age it relaxes more and more but usually not significantly until they are older.
When he is old enough to pull the skin back by himself is when it should be done not before and certainly not by anyone but him.
I forgot to add be sure that any time you see that the foreskin is retracted that you gently pull it back over the glans. If it gets stuck back there (rare but possible) it can be a very scary and dangerous thing.