All pretty, pretaaay, pretty good.Questions:
Hows the movement on his fastball?
4-seamer or 2-seamer?
Can he throw it consistently for strikes? (Rookie leaguers could be swinging at anything close)
Can he throw it for balls? (For when he needs to pitch around someone but stay close to the zone)
Can he throw it for strikes or balls without tipping the pitch type or location?
Does he hold his velocity into the later innings?
And that's just the fastball. He's got other pitches as well to learn to throw consistently well. Sounds like he's put in a lot of work and it's paying off, but again rookie leagues are a far distance from the majors.
What about holding runners on?
How's his move to first?
How's his move to 2nd?
Can he field his position well?
What are his feet like while fielding a bunt?
How does he handle come backers to the mound?
Does he go to the correct spot to back up wherever the throw is coming from?
Can he bunt?
All this is stuff we don't even consider when we see someone lighting up the radar gun. But we are well served to keep in mind that baseball at the highest level is really, really, really hard and a lot more goes into it than a fastball that sits 94 and touches 97. And that's just in rookie leagues. There's entire books that can be written on the subject of intra game strategy between the batter and pitcher that we lay fans don't have a clue about. That doesn't come until much later. But the margin for error is miniscule.
All that said, there's no reason to think Santillan can't learn all that stuff. Seems to have taken to the coaching concerning his mechanics, so no doubt he'll be working on those other things as well.