Hi Spandan,
Nice to know that child is doing very well. Children having 3 limbs hypertonicity always give hard time. With these children main problem is trunk hypertonicity. Try to improve functions at shoulder first. Try to take arm in tone inhibiting pattern at shoulder. Scapular mobilization is always helpful to start with to bring shoulder in abd and ext. rot. Take the child in sitting on bench, using the shoulder as KPC in abd and ext rot, encourage the child to touch or pat the objects by actively extending the elbow and wrist as much as he can do it easily. Use weight bearing on hand at sides or at back keeping shoulder in abd, ext rot and extension, elbow straight, and wrist extended. As the tone decreases and arm gets active, encourage the child to do bilateral activity in front rather than emphasizing to do much activity with only affected hand. I usually ask parents to mobilize the scapula, keep arm in TIP and than encourage the child to hold the arm in front with abduction while putting on sleeves during dressing. You can use hand painting using affected hand keeping the worksheet in front on board/ wall so child has to stabilize shoulder in flexion and abduction.
Plz let me know if anything you cant understand because of my writing. These days my centre is closed for winter vacations. But will try to post some pic showing some excs once centre is reopen. Let me know if you want anything in particular.