I would not worry one whit about it. Substituting “W” for “R” or “L” is one of the most common articulation errors we associate with very young children or “baby talk” (“wabbit” instead of “rabbit”; “wittle” instead of “little”). From your other comments, it is obvious your son talked very early and is a bright and eager learner. He will undoubtedly correct this on his own in the near future. If not, and if this is the only substitution he has, a few sessions with a speech therapist will probably eliminate the problem. Making the “R” sound is much more complicated than making the “W” sound. Make both of them slowly yourself and feel the parts of the mouth and tongue and lips that are involved. Making the “R” is pretty much inside the mouth, and a child can’t see how it is made. Pronouncing the “W” is much easier. And try not to correct him too often; the last thing you want to do is make him anxious about his speech.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.