Take him to your local bike shop and get them to help you pick out a bike.  They are interested in getting him a bike that will fit him and are experienced in the industry.  Don't buy from WalMart, or Academy, or department stores, where the sales help is usually clueless.  The average "department store" bike gets less than 100 miles on it before it hits the dumpster - and for good reason - they aren't fun to ride because the quality is so poor.  Even for a kids' bike that you expect him to outgrow in a couple of years can be handed down to a sibling or friend.  You'll pay a little more, but the piece of mind knowing that his bike will hold up to the crashes, getting dumped in the yard, jumping curbs is worth it.  \n\nYou can get good kids' bikes in some of the main brands - Giant, Trek, Specialized, etc.  They will be good quality.  (Note, if you are as old as I am, Schwinn was the thing to get when you were a kid - but they are no longer.  The name was bought out by a very low end, cheap company (Pacific Mongoose) and the quality has gone way downhill.)\n\nAnd as everyone says: get a helmet & a pump.  The tires should be aired at least once a week (they will lose ~10% of their pressure *each* night).
how do I pick the right size bicycle for my child (45 inches, 4 years old, needs training wheels)?