Is it normal for a mother to apply lotion to a seven year old boys private areas after a bath?

It would depend on whether the child can do it for themselves. Some boys dont do a good job of cleaning/drying/powdering or what have you at that age. Personally I have never heard of a boy needing lotion to be applied to his private area unless there is some kind of a skin rash or problem.

The bigger question is what makes you suspect that it is not normal?