This is a pretty common problem with singleton pups. Most dogs start learning that their teeth hurt and they have to control their jaws to be allowed to keep playing from their siblings, so when you have only one in a litter, they fall behind on that learning curve. They can still learn, of course! But be prepared to put extra work in.
Figure out what your dog likes to chew. Safe chew toys can be rubbery or hard (usually nylon), textured or smooth, maybe with holes or grooves to stuff food into to encourage chewing. There's even some options like the Dogwood toy, which is safe nylon mixed with real wood particles, so it smells and tastes like wood but doesn't splinter like a real stick (my younger dog's favorite, and saved our furniture, haha). Always have a toy around so you can give that to the puppy as an alternative to your feet, hands, etc. Praise and fuss over the pup chewing a toy instead of you, whether or not you had to encourage him to make the right decision. Soft/rope toys can work too, but they should only be given when a puppy is supervised, since they're easier to destroy and take apart in chunks that can be dangerous to swallow.
When the puppy decides he'd really rather chew on your foot and can't be persuaded otherwise, leave. This may require setting up some baby gates or a pen you can step over so the puppy physically can't follow you. Wait five seconds, return. If the biting starts again, you immediately leave. Rinse and repeat. Essentially the idea is to teach 'you don't get playtime or interaction with the humans if you bite'. It'll take time, especially with a singleton, and you need to be consistent, but puppies do get it eventually.
You might want to do it in stages, by first allowing the puppy to mouth you, and only removing yourself when he causes actual pain, then move to pressure, then teeth on skin/clothes at all. This has the added benefit of teaching more nuanced control over their jaws than just "biting or not", so if they ever do bite as an adult (out of excitement, fear, pain, what have you), they are far more likely to bite with restraint and do less (or no) damage.
And lastly, if the puppy just seems absolutely wild and unmanageable, ask yourself when they last slept and how much they've slept total that day. An overtired puppy is a terror, just like an overtired toddler, and if he's been up for a while, there's a good chance he needs to be popped into a quiet area like a crate or pen until he's had a proper nap.
