Unfortunately, it appears that the only eBooks available are bootlegs available on some really iffy sites. Also with the amount of hard bounds Silverstein has available, I would guess he just changed publishers. That could be the reason for no eBooks at this time. But I am just speculating on this.
Shel Silverstein's books may be caught in the same sort of legal limbo as many other recently deceased authors (Salinger is one example). The original publishing contracts do not cover electronic publishing, but if his estate is still in probate, and that can take years, there is nobody with the authority to sign a new contract.
My favorite book as a child was one of his first, much lesser-known children's works:
Try the "How to Train Your Dragon" series by Cassandra Criswell. My 10 year old can't get enough of them ( and the 10th is coming out soon). Not like the movie, but funny/boy humor type of stuff. Good luck.
I've launched a new Bible Camp Mystery series for 10 to 14 year olds. The first one now is available in the Barnes and Noble Store. The second one and possibly third are expected laster his year. You can find it here: http://www.barnesandnoble.com/w/lost-in-the-woods-cheryl-rogers/1114304247?ean=2940044256613
It was written with boys in mind...