I agree it doesn't look much like hand, foot and mouth I'm not sure if mumsnet is the best place to get a proper answer. Would you be able to take your DS to a see a pharmacist? They would be able to advise whether it is something that can be treated with an over-the-counter cream, or whether a GP appointment is necessary.
I agree doesn't look like HF&M. Ring worm possibly. Has he been running around barefoot outside? It doesn't look exactly like a tic bite but it's the only other thing I can think of. I'd try to get to a pharmacist or dr to find out.
My mum had a look (retired doctor) and says this is neither hfm nor ringworm It's an urticaria rash which could be of no importance or part of a more systemic problem especially if any thing in mouth or confined exclusively to legs. She suggests seeing a paediatrician if it is still active.