My first suggestion is to not worry about the r sound. My daughter was in speech therapy starting when she was 3. She needed help with her articulation. With help, she learned all her sounds except r before she started kindergarten. The speech therapist worked with her on r for a little while, but my daughter just got frustrated. After a month or so, the speech therapist suggested that she was ready to "graduate" from speech therapy. She said that many kids can't make the r sound at that age. It is physically difficult for them. She also said that with only the r sound missing, my daughter wouldn't even qualify for speech therapy if she were tested again. My daughter is seven now and has mastered the r sound on her own, without any guidance from anyone.
That being said, if you are determined to teach your son how to say an r sound, I would try to focus on saying it at the end of words. It is generally easier to master in that part of a word. After he starts saying it at the end of words, you can start having him say words slurred together, like airrrobin, or earrrrun. Then gradually have him separate the two words. It might help him to then say the r at the beginning of words.
If you think your son's problem is severe enough, elementary schools offer speech therapy services to preschool kids free of charge. Contact the school district and find out what you have to do to have your son tested. They will be able to tell you what kind of help your son needs.