My suggestion is passed on from experience of several surgeries to my young horse at Newmarket. The vets suggested a high quality pro biotic for 30 days post op, it made a big difference to blood results showing an improved immunity. Generally speaking you know if it is high quality if it is around £30, anything less and the majority of the pre and pro biotics never make it to the digestive tract for proper absorbtion. Good luck
You can get an injectable immune-buster from the vet. I have a little foal who moved here on livery at 2 months old without his mother, and he has really struggled at times. I was pretty concerned about him on a couple of occasions and suggested to the owner that she might want to have the vet come and give him a bit of a jolt. Vet left me with a few injectable immune-busters - they were basically filled with vitamins and minerals. They did work, the foal has really picked up with them.
There is some excellent advice on this thread for you to consider. In my experience, a vit injection from the vet has had excellent results with a couple of my rescues and another horse on the yard which had severe anaemia.
You could also PM H's mum who developed a suppliment called Coligone which I have put one of my horses on. If you read her website it will show you that many vet surgeons use Coligone pre and post op, it contains ingredients which will help to boost recovery.
Vets can prescribe drugs specifically for boosting the immune system if they think it is necessary, ie to help fight off an ongoing infection etc etc. Some are injectable and some go in the feed.
For just general well being you could try Pink Powder or Protexin Bio-Premium . Red Cell/Pro-Pel is a blood booster and rich in iron.
Garlic is supposed to be good for the immune system too.
Global Herbs has quite a good website for info on herbal remedies I was having a look at this the other day.
I hate to sound so boring but if it is for a specific need/problem I 'd speak to the Vet for advice!...
Would agree with sassyequine re the probiotic, William is on one given by the vet after his op, and it does seem to help.
For a general boost Equine America do a good product, the name escapes me but they will tell you I'm sure.
After our mare had a severe virus she had probiotics and then gallons of Aloe Vera. Seemed to do the job, but a good thing the insurance company was paying!