How long have you had him? What's your temps and humidity like? You said he was eating regularly before and this was with you, or you and the previous owner? How many meals have you successfully fed him?
If you heat and humidity aren't correct, I suggest you listen to him breath to see if you can hear continuous popping sounds or wheezing. If he's getting sick , he may not eat.
Also, he's about 5 foot long and in a 55 gallon? I don't know tank sizes, what's the approximate length and width of the tank?
Another thing...and I have NO idea but it's just a thought even though he's quite young and I hope more experienced retic keepers can tell us... but is it possible he could be feeling the need to breed early?
That being said, make sure your temps and humidity are perfect. Also, don't handle him again until he does eat and even though I'd make sure he ate at least two consecutive feedings before handling. For cleanings, place him in another tub and quickly clean and put him back. I also suggest waiting until he see him more active and appearing to hunt. Then try offering food at night instead of the day. If you feel you must offer, offer every 10 days as opposed to every 7 days. Personally, I'd wait and see if you can get a more natural response out of him by letting him tell you when he's hungry. Sometimes we get too anxious when our snakes don't 'act' like we think they should. But I"d only worry if he started loosing weight.
Another suggestion is see if another food source will entice him into a better food response.
Good luck and I'm sure more experienced keepers will chime in with different or better suggestions!
And if you ever get at your wits end with him not eating, you can always send him to me.

He's absolutely gorgeous!