Mark
The clip is actually about par for the course for the majority of pitchers this age so it is good that you are sincerely seeking help before internalized to the point of no return.
One thing that was mentioned by several others that is glaringly obvious is the lead foot>
Landing of lead foot should be toe to heel and not heel to toe. This has to be corrected and soon. How do you do that? This is what I do with pitchers that I have helped correct this major pitching flaw in delivery> Get an ole coke crate or box the same size in height and width as coke crate. Have pitcher practice stepping over crate from sideways position as in stretch over narrow side of crate with crate turned face down(open side down). Forget slide-step for now because the crate will be too deep to allow this and not purpose of drill anyway. This drill can be done from stretch windup or full windup but better as I described from stretch windup. Lead foot will be forced to land toe to heel in this drill and correct movement will be internalized in muscle memory after a while. Some faster than others. Drill will also force lead foot to land in more closed position as should be as in almost 45 degree angle when toe to heel action occurs in landing as pitcher is forced to step over crate.
Would give examples of successful pitchers that I have worked with who are in Major Leagues today that have used this very well kept and guarded drill but not in name dropping mood today
The second glaring lowerbody flaw is the back pocket needs to be more visual by hitter
when bringing front leg forward. This can be easily accomplished by pitcher consciously thinking about keeping front side closed as long as possible when coming forward and pitcher can also work on breaking hands a little later as also mentioned.
Could also try to be a little slower and methodical overall movement in full wind-up. Also, as mentioned by HS Coach, remain in contact with rubber in side step of full wind-up.
This is a start and hope suggestions get this fine young pitcher on the right track.
Shep Does Care