You can download pretty much Amtrak's entire rolling stock on this website:
http://s4.zetaboards.com/amtkz/pages/rs_dl/
Amtrak locomotives cam be found here on the same website:
http://s4.zetaboards.com/amtkz/pages/loco_dl/
JCitron is right about the locomotives you need. Jointed Rail has quite a variety of locomotives your son wants. Most of the stuff is payware, but they have a lot of freeware. They occasionally 'retire' a piece of payware, meaning it becomes freeware. Note that you need to register, which is free, in order to get the stuff you want.
RRMODS also provides a fair amount of freeware and payware your son would like. However, some of them have problems with them, but most are easy to fix. Registration is also required for this site.
Also, on the Download Station, Chris (n8phu) has managed to fix a large amount of locomotives, most of which your son would be interested in.
You can also find a small amount of METRA stuff on the Download Station, just type in 'metra'.
Please note that you must NOT click on 'download station' on trainzportal.com, as this is the new version of the Download Station, and does not have the vast amount the original Download Station has. This is the Download Station you must go to:
http://www.auran.com/TRS2004/DLS.php
This should get you started in finding what your son wants.