start w/opthalmologist at that age. an optometrist won't be able to do the extensive testing needed to determine if there's an underlying issue or if he just needs vision correction. If its just vision correction, you can switch after the first visit.
you have a whopping 2 choices in Denver for pediatric opthamology - Children's Eye Physicians (multiple locations in the metro area) and Children's hospital.
I've not been thrilled w/Children's Eye Physicians so we're trying Children's hospital for a second opnion next month for my little girl (now 5 1/2). For a pediatric practice, children's eye physicians is not very welcoming of kids or of kids being kids - they've had little patience w/my little girl being scared (we started when she was three) and I'm appalled at the waiting area at 52nd and Ward - almost no toys or kids' magazines - they seem to want you to plop them in front of the TV that's always running. I understand their using the TV during an exam because most kids are so entranced it keeps their eyes focused where the docs need them to be but if you have kids like mine who don't take well to the TV being randomly (in the kids' minds) shut on and off, it's rough on the kid.
That said, you should go to a pediatric practice, so you take what you can get. Note that my pediatrician uses children's eye physicians for her son and isn't much happier than I am but again - there's really no choices. We've seen DeSantis and Ruth - both are decent docs.
I've been much happier w/my optometrist for my son - but he has no issues - vision is perfectly normal so we're just checking every couple of years. My optometrist treats my son as a real human - explains everything he's doing before he does it, includes him in the conversation, etc.
good luck