Aw, poor little snookums!
The best thing I can suggest is to find out when your local version of the RSPCA is open & (if they accept lost animals) get your son to catch it & deliver it there. Also try animal shelters in your area in the yellow pages. There should be one of these open on Sundays.
Taking it there is a better option than just shooing it away as the critter is likely to reproduce, say nothing catch diseases & spread them around. A real problem with strays. Bloody humans who don't neuter their cats!
OR you could consider keeping it!
Assuming your son is willing to look after it & you're willing to pay vet costs to get it checked, inoculated against diseases, etc .... This could give you a warm glow!