All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
In principle having children is a right for both spouses; therefore, it is not permissible for any of them to prevent the other from this right without a sound religious reason.
Hence, if your husband has a good reason supported by the statement of specialised doctors that becoming pregnant in your condition [being of little weight] may affect your health, then he is right. However, if he prevents you just because he is not willing to have many children because of the issue of providing for them, then he is not permitted to prevent you. For more benefit, please refer to Fatwa 84472.
Finally, we advise you to discuss the matter with your husband in a nice manner in light of the above.
Allaah Knows best.