Lawful non-immigrant status are H1, H4, H3, H2, F1, F2, M, B1, B2, L1, L2, J, C, BCC (see list http://travel.state.gov/visa/temp/types/types_1286.html).
Lawful immigrant status is Green card.
So your husband doesn't need to get Selective service letter because he entered USA on his H1B...