What you describe is quite part of a pattern. The parent who spends less time becomes more valued and also because he gives presents and treats. Be glad that your son has a good relationship with his father. This is fortunate.
Please do not start competing for your child’s affections. Your child will definitely know that you love him and that you will be there for him. (You could also say so in words, once in a while). He calls his father to feel assured that he is also there.
If you have access to a Counselor or Psychologist, do feel free to consult her. I do not think that you or your son seem to be in need of professional help, but sometimes it helps to see your own situation objectively.