Diet change is not typically a mode of treatment for low platelets. Hopefully the underlying cause of the low platelets has been identified--this cause may require a diet change. Talk to your veterinarian to gain the best advice and treatment plan for your dog. Do not give your dog garlic--this can be toxic and is dose dependent. Large amounts of garlic can cause low red blood cell counts.
I would try to increase the whole grains and brown rice in his food. Make sure to avoid adding sugars and dairy to anything that you prepare for him. Also try to make sure the food is blended well to help with digestion of the food. Talking to your vet is always a good idea to make sure you are feeding your dog a balanced meal that will satisfy all of his daily requirements.