Kari, if you use the "search" feature at the top of the page and enter "ATG" you will find lots of first-hand descriptions and discussions. Most of them will be for Aplastic Anemia, however.
ATG isn't used nearly so often for MDS unless it's of the hypocellular variety (most MDS is hyPERcellular), but there have been studies showing response in a small percentage of RA and RARS patients. Most of what I've read is several years old now, and the samplings were fairly small, such as this report:
http://www.nature.com/leu/journal/v1.../2403124a.html
What type of MDS does your Dad have? Have his doctors explained why they think this would be appropriate for him? It's possible there could have been new developments or new thinking on this.