I do not think that being a pen pal is a good idea. I think it would open the door to allow him to talk and be open with strangers. When he is old enough It might encourage him to go and seek websites where people chat freely. Those sites are unsafe for almost everyone.
(Sex offenders) roam those sites looking for prey.
If you want him to write more you can encourage him to write a story and illustrate his pictures and then you can have it turned into a book.
My daughter did this for two years in a row in the public school system.
Here is the company's name:
Nationwide Learning, Inc.
PO Box 5263
Topeka, KS 66605-0263
" Motivating Students To Learn Through Publishing".
http://www.nationwide-learning.com/
When we did it I think it only cost about $7 or $8 dollars a book, we ordered a few extra books to share with our family.
These are just my thoughts.
God Bless