Can we apply for my husband's DTV and add dependents later?

Yes

Dependents apply separately. Once the main applicant is approved, you submit a new application for your son under the dependent category, proving the funds, that his dad has a DTV, that he is in fact his dad, etc. and then submit the application fee and wait within that jurisdiction to be approved. Then head to Thailand once approved.

Yes. You don't really add dependants, they each apply for their own DTV quoting the principal DTV holder

Yes