Exclusion of battery weight from the total weight still does not do away with the collpasibility (I think disassembly is intended here) requirement. The wheel chair can be any weight, but it must be amenable to disassembly into parts, each weighing less than 60 Kg.

I can understand that disability is not the boys fault, nor that of the mother, and that people with disability should be offered as much assistance as possible, but it does not absolve travellers with special needs from the responsibility of informing the airline of their needs.
Is the 69kg limit on a single item in contravention of EU laws?