His weight goes up and down according to the chemotherapy he is on and the steroids that put on a lot of weight. His is a special situation. Think nutrition not weight, for now. He needs protein daily, and easy sources are eggs and dairy products because they don't require much chewing or effort to eat and are very versatile. Nuts and dried fruits pack calories and nutrients into small volume.
Supplementary shakes can also help, as can a thorough multivitamin.
Ask your doctor to recommend one. I know very little of your medical history, so you should ask your doctor this question. You may be at increased risk for gestational diabetes, which can be dealt with during pregnancy, but requires close monitoring if it occurs. My 14 year old daughter is beginning to really gain weight and is not very active outside of school. Let her choose activities she likes so she'll stay motivated.