Well you could use brass, but lots of steel track and switches can be found on Ebay pretty reasonably, so if indoors is all he does then all steel is OK. I have seen several large indoor layouts using all steel alloy track.
If cost is not the problem then you could fork out the bucks for all brass, and peddle the track you have on Ebay. Or, As I said they could be mixed temporarily using the track clamps. If the track is taken up and down over the year, brass is probably best overall, and would allow setting up outside if desired.
Lots of options, but you will have to decide which way works best for you based on you situation! Hopefully you are having a great time with him and his trains!!!!!

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Bill