Thanks for all the replies. All been very helpful.
So we finished the course on Wednesday (never putting me or him through the equi haler again!).
He can now walk, trot and canter across the field without coughing, but will cough once or twice after eating his bucket feed.
I'm new to this, so wondering if this is normal/to be expected for an asthmatic pony or am I aiming for zero coughs? I'm thinking of my own asthma which is generally well controlled but I still have a bit of a cough occasionally.
I've also just gone on to prescription antihistamines because of the early pollen build up, and I do think it affects Bo too. Currently giving daily cetirizine which seems to be helping.