Looking for advise on allergies and getting a HEPA Air Purifier?

My son has allergies and the doc is reconmending an air purifier.Do they work, would you reconmend getting one? Not sure what I am looking for when it comes to features on them and don’t know what brands are good. Also I have heard about people installing some sort of purifier on their furnace. How much does this cost and does it work? Looking for any thoughts or advise. thanks