Too early to tell. He'll likely regress in his sophomore season.
I should expand on why I asked. Anyone can regress, so Slavin certainly may. But, I was curious why you think it's LIKELY. If it's simply "because some other players regress their 2nd year", then my view is it's just as likely he won't regress as that's not a good reason.
Here are some reasons as to why he may be less likely to regress:
1) Slavin isn't coming into the NHL as an 18 year old or 19 year old like some guys do (ie..Hanifin). After being drafted, he spent a year in the USHL and 2 years in NCAA so he's already 22 and a bit more mature.
2) He's already filled out and is pretty good sized player. He was 180lbs when drafted and is now 205 so he's "grown into" his body.
3) He's pretty well grounded in that he's already married and a religious person. He's not the type that will be going to be going out partying with is new found wealth and fame.
4) Sometimes, guys regress because they get easy matchups as a rookie, but the get tougher matchups in subsequent years. That's not the case with Slavin. When it comes to quality of competition and defensive zone starts, Slavin was 3rd on the team (in terms of more difficult) and only barely trailing Faulk and Hainsey. Plus, when Faulk was injured, it was Slavin that moved to the top pairing (on his off hand side) and stayed on the top pairing for the remainder of the season, even when Faulk returned (Slavin moved to the left). He's not going to get much, if any, harder assignments than he received this year.
5) The type of game he plays, it's kinda "good at most things". He's a good skater, he's a good passer, he's a good puck handler, he's strong, he has good gap control, he's good on both sides, he has good instincts, etc.. His shot is probably his weakest point right now. It's hard to "take 1 thing away" from a guy like that, which can lead to regression.
Of course, there are also reasons he could regress. Maybe the pressure of expectations get to him next year, and maybe teams focus on fore-checking the hell out of him, he was relatively healthy and maybe he gets injured, etc...etc..
I'm not saying he won't regress, as he could. I just don't think there are enough valid reasons to suggest it's "likely".