I think some of the suggestions from Kat_Eden are very good.  I would add this:
You might want to ask the parents' permission, and get a formal "release" to consult with his school in Florida, if you have not already done so.
Synchronizing what you are doing with what is happening in school might help by improving the consistency that this boy has within his learning programs.
You might also discover some effective strategies they are using, that might help you.

It sounds like you have found a few reasonably helpful strategies. These kinds of issues do not resolve overnight, and sometimes there is a bit of trial and error.
I also recommend that you talk to the parents about making sure that whatever reward system they use, they remain consistent.

One other thought.  Have you considered reducing the time to 20 minutes?
Or 15 minutes with a break and then another 15?  The 30 minutes might be too much for such a little one.

Also, it is only a month or so. That is not a lot of time for a child so young to get used to a new situation. Also, it sounds like you must be using "skype" or some such medium to work over the miles.  Given all that, you are not doing too bad, especially since you report that things have improved somewhat.

Bette J. Freedson, LICSW, LCSW, CGP

Hello,

I don't know the details of the "behavior plan" you've implemented but I've seen positive reinforcement "sticker charts" work really well for kindergartners who have a hard time focusing and paying attention to their work.

There are two options here: The first is to keep it really simple, just create a chart with lots of boxes - it will basically look like a calendar page but with no days across the top, just numbers in the boxes.

The second is to make the same kind of calendar chart but put specific behaviors across the top (where the days would go). These can be things like "Sitting still for 10 minutes" or "Finishing your work without being reminded".

Either way, let the student know that you, his mom, and his step dad can all give him stickers for great work. When he earns X# of stickers, he'll earn a reward. (This needs to be something appealing to him - going out for ice cream, choosing what's for dinner that night, getting a new book at the bookstore, etc).

Keep the chart taped right to his work space (desk) so he can see it fill up! Every time he earns a sticker the adult who gives it should say something very specific about why he's getting the sticker like "Great job! I'm giving you a sticker because you were able to sit in your chair for 10 minutes in a row while we worked on this lesson. I'm really proud of you!", or "Good work, you were able to complete that lesson without being reminded to keep going. That deserves a sticker!". When you "catch him being good", tell him what he's doing right, give him a visual reminder of his good work (the sticker) and give him something to look forward to (the reward) you may find that he has an easier time staying focused.

Another trick I've seen work well for "wiggly boys" this age is to buy them a fit disk (link below). It's similar to an exercise ball but you use it in a regular chair. Wiggly boys can get out a bit of their energy by moving around on the chair without actually getting up. They're not too expensive and can really help with focus.

Hope this helps!

Kat