Another thing I'd note - and I don't think anyone would argue - is that an SSD is a *crucial* part of the build. The one component above all others that improves user experience across the board imo is an SSD. This includes game performance, as games spend a lot of time loading data, textures, etc from the hard drive while you're playing. You'll get a lot less jitter/pop-in/etc running an SSD... and that's not even to speak of the 6-7 second boot times and the generally better experience you have in day to day use around the operating system, either.
Again, this is an easy thing to expand on later, but I would take a 250gb SSD in a gaming build over a 3TB HDD any day of the week. It's cheap as chips for your son to mow a few lawns and save up for an extra stick of ram or a new $70-100 HDD... you want to set him up with the most valuable, difficult to extend components when you're on a budget. I would prioritise the GFX card, the CPU and an SSD above all else. Everything else is either cheaper to extend or just window dressing.