My farrier highly recommends aspirin for horses in the early stage of founder. My vet prescribes nitrous oxide application which I have used for early-stage metabolic-type (vs. mechanical) laminitis with 100% recovery rate when combined with complete restriction from all grass and grain products.
The cause of your horse's founder and the length of time that has passed from the beginning of symptoms may affect how your horse would respond. Drug recommendations and dosages can be obtained from your vet, but for further information there are several web sites that might be helpful:
Our vet had us give our mini mare an aspirin twice daily, we now have her on thyrod meds and trying to get her set with regular farrier trims. She is doing some better but haven't really had her on the meds long enough yet.