Looks like a skin cyst of some sort, like a human sebaceous cyst?
It's probably nothing but I guess if it doesn't go after your prodding, best to get vet check......
maybe a grass seed, or sebaceous gland that's got a bit clogged? Like Jac says, it's probably nothing to worry about but a vet visit to double check might be reassuring.
My dog Jodhi used to have something similar on her head that would come and go. After checking with the vet, they told me just to drain it myself. Hopefully it's the same for Stanley and no big deal at all.