Hi S.
If the child is terminal then you are in a different situation than we were in.
We have been on both sides of this issue. The healthy child visiting by email, or in person is the easier place to be.
Because of your question I will answer from the sick child situation. My daughter was in the hospital 28 days. It was the longest 28 days of our lives. The bright spots were when we heard what was happening in the outside world. In hospital you don't know the weather or news, but it is good to hear those things. The emails at the hospital were just as bright as mail call when cards are brought, and it is sad to see no one has written to you.
Lots of short notes were better than one long note. Every day "hi, how are you? I am doing -- going--reading---whatever " Ending of course with feel better in some form or other like it was a get well card. They were always our favorites. Our daughter was not always strong enough to stay up long and read lengthy messages. Your situation is that they are young enough that it will take lots of concentration to read even short messages.
We loved happy upbeat what's going on messages.
Don't discourage, encourage because as he gets older you want him to care about others needs and right now he is so build on it.
By the way I stayed at the Ronald McDonald House and the letters I got encouraging me were a great blessing too even when they were not people I knew well. Don't discount that and suggest you let mom know that you welcome suggestions about what your son can say to hers that will help and to be sure and let you know what he should not say.
Our friend hated to get cards that said "get well" because the diagnosis was terminal. I praise God we were not in that situation.
Hope all will be well soon so that he too can be out there in the snow sledding, or maybe playing T-ball as the season changes.
God bless you and the wonderful outreach
K. --- SAHM married 38 years === adult children == 37,33, and twins 18.