What are the risks of spinal tap on a two yr old boy with developmental delay?

Minimal: There is nothing in medicine without risk, but those associated with lumbar puncture are minimal. There is a very small risk of infection, tear resulting in spinal fluid leakage, and the most common is headache, which can be moderately severe when it occurs, but is self-limiting and vanishes within several days.

Little risk: Done by an experienced physician, there is little likelihood of danger in having a spinal tap.

Quite safe: in experienced hands, across the board any procedure carries a small risk (1-2%) of infection/bleeding in the best of hands in the best centers of excellence, your doctors know the risk/benefit ratio, and definitely there's a good reason for resorting to spinal tap, wish yourself and your son wellness