Just a BIG piece of advice to share with you for your son's safety...
NEVER let him ride his bike WITHOUT shoes that wrap around his heels...NO flip-flops or other "loose" type shoe, and preferably have him wear tennis shoes that tighten up with VELCRO as opposed to Laces that tie....Laces come untied, and can get caught in his spokes or the chain.
I can not for the life of me figure out "why" parents keep insisting on the helmet and knee/elbow pads, but NEVER talk about what type of shoes our kids should be wearing to protect their feet when they are riding their bikes! (I am the victim of such an accident when I was 4 years old, and have been suffering the consequences of it for the past 38 years. If anyone really wants to know what happened, please email me at ____@____.com...but I will warn you, it's graphic!)
As for whether to get a tricycle or a bicycle...most kids at your son's age are learning how to ride bikes with training wheels. Usually the trainers will come off sometime between the ages of 6 and 7, depending on the confidence and ability of the child. However, I believe I have seen some bigger trikes designed for "older" kids (4-5), but not very often. Lastly, do not buy a bike for your child to "grow into"....another safety issue.