He has done okay and is getting better all the time. Why was he let go?

Depends on who you believe!
He certainly didn't seem to be rated by Mattersonn who consistently picked him as 18th man even when we picked up injuries. Myself, I always thought he looked like he could develop into a good player and was disappointed to see him leave. However, with Matterson's ability to restrict the progress of young players this was no surprise.
There are those that claim Ryan was offered a decent contract but chose to join Salford as he thought he would get more game-time.

Which has certainly been true - I've been impressed from the first time I watched him in a red shirt - and I think he is steadily improving - a first rate acquisition for us IMHO

perfect replacment for stappo in my eyes like said above didnt have the best of starts but as soon got to grips with life with salford putting in some good solid performances and not to forget boyle is only 23

son of susan .started not to clever ,but now starting to show how good he can be ,hopefully in a red shirt for years to come

Some questions over he temperament weren't there?

Recently spoke to one of his ex-Cas team-mates who said he was " as mad as a bucket of frogs" but a good lad, a hard worker and definitely no trouble.

I've been quite impressed with him as a young lad in a position where most don't excel until their latter years.