how to request a change in school time?

ToastieSimons

DS1 is in an afternoon half day program.

He is so close to walking and today I realized that him staying home with me a half day and at school a half day is not going to give him the physical fufillment that's going to get him to walk.  We're in 800 sq ft, there is no room for him to really practice walking.  On top of that DS1 has PT 2x/week, Ot 1x/week and DI 1x/week and we're adding in ST soon.  So it's not like I can give him undivided attention for those 4 hours a week.  

When we started the transition to the school district I asked for a half day preschool.  I don't think him or I were ready for a full day.  Our social worker said that it can be increased at anytime.  I called and left a message and the secretary asked me if there were any openings in the full day preschool.  

Do I have to request this in writing?  Any suggestions?

DS1 is in a private therapeutic school that the district placed him in.