Question on proper use of "to" versus "for"?

In that situation, "to" has more of a sense of "at" (as in directed toward), whereas "for" has more of a sense of "to the benefit of". I would most likely use "for" rather than "to", since the inspiration is not purposefully directed at the son, but instead is simply something that happens to provide him a benefit.

It is quite possible to be an inspiration to your son, but it seems to me that your sentence is more intended to mean being an inspiration for your son.

Either works - I would incline to 'for'

but had it been something like -

"your return to school has been an inspiration to us all"

'to' would clearly work better than 'for'. It is more a question of style and usage - ie which sounds more 'natural'.

You can always hedge: Going back to school is also inspiring your son.
But as Mrs Emmeline Lucas would say, "I never hedge. One thing or the other."

For is a better word but both would work. I would try to explain the 'English rational' but it would suit both 'to' and 'for' as both are correct.

On the surface, they both sound correct but if you make an in-depth analysis then the word 'for' is right in this context.

eg: doing something 'to' someone. (doing your duty)
and
doing something 'for ' someone. (doing willfully)