Your DS is too young yet to start solids. His belly can't handle it til around 6 months old.
Breast milk (or formula) is the vital source of nutrition a baby needs til 12 months old. It should always come before food is offered & a baby should have either breast milk, or formula til at least 1.
I agree with your DH. I'd try to keep up the breast feeding for as long as you can, but with solids as well from around 6 months