As long as your son can tolerate Curd there is no harm in giving. Don't give him chilled Curd
Always give him curd At room temperature. In winter try to give hot meal,along with curd. then no prob. Cold curd will affect our throat & cause cold & chest congestion.
As a kid I've always eaten cold curd in all weathers and not caught a cold. I guess a lot depends on where you live and how your son's body copes with it. If he loves eating curd you should let him.
It's absolutely Ok. My grand son also eats both in lunch and dinner. My father in law age 90 eats regularly in dinner.
In fact yoghurt is better than milk as it has probiotics which are good for the guts.