Don't wait. A good occupational therapy evaluation, preferably with someone who specializes in babies, will give you a lot of information and will be fun for your son. As often as not, this type of visit is 100 percent reassuring--you'll get tips for encouraging and recognizing normal growth and development. If there is a problem with your son's motor skills, early diagnosis and therapy can make a huge difference.
I feel the same way when it comes to questions about a child's hearing, vision, language and more: Trust your doctor, but trust your instincts more.