Hi,

2d HomMM question of 2day, what a coincedence :)

anyway download the offline installer and install the game on both pc's and you should be good to go for local play

i belief you can also just copy the directory to a usb drive and use that to copy the game on your sons pc, should work fine

if you get into some connection errors you might want to launch a direct question with your problem on the forum or try and see if a search on google will turn up anything

Good luck