بسم الله الرحمن الرحيم
You should have cut all the hair of head that day. However, now cut all the hair of head and give silver in sadaqah equal to its weight, it is mustahab and bury the hair. If you cannot give silver in charity then too do not delay in shaving the hair. You can shave the hair by razor, blade or trimmer which could clean the whole head provided the child does not feel any pain.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.