Also - there are known negative interactions of that mutation with other diseases.

From wiki:

CCR5 Δ32 can be beneficial to the host in some infections (e.g., HIV-1, possibly smallpox), but detrimental in others (e.g., tick-borne encephalitis, West Nile virus). Whether CCR5 function is helpful or harmful in the context of a given infection depends on a complex interplay between the immune system and the pathogen.

What he did was flat out unethical and completely unjustifiable. If he had discussed the risks and benefits with the parents, ran it past an IRB, then maybe we could be having a conversation about the ethics of “playing god.”