My son is 6 mos old and has eczema on his scalp, what is the best thing to help him stop itching?

Proper care. Itching is a part of eczema. Once eczema is under control, itching should abate. In case of severe eczema, antihistamine is often prescribed in addition to steroid cream. Consultation with your baby's primary care doctor is advised.
Cradle cap. If it is just on his scalp i would recommend using creamy baby oil, if not better then i would recommend dandruff shampoo once a week and massaging his scalp with a soft brush.