Thanks so much for your question. There are lots of homeopathic remedies that are useful in ITP (and most other blood, and other, disorders).
The selection of the correct remedies in a potentially life-threatening situation such as this needs to be carefully individualized. ITP is not due to an acute (although it may have come on acutely) immune imbalance that is amenable to one remedy or another (like Arnica after a bruise or injury). I therefore can not recommend a specific homeopathic remedy.
Ideally, you would add a veterinary homeopath to your veterinary healthcare team. S/he will guide you in appropriate integrative therapy using homeopathic medicines as indicated.
In the meantime, I'd concentrate on helping his body heal by strengthening it through a fresh food diet, plenty of fresh air, sunlight and exercise. Vitamins C, B12, antioxidants (e.g., a grape seed extract like Antiox), Coenzyme Q-10, omega-3 fatty acids, etc.
You can find certified (and non-certified but trained) veterinary homeopaths at: http://bit.ly/w6iV9I
Good luck! Please keep us posted on his progress.