Sorry the HP did not work! Glad you reacted quickly and Ian seems to be doing well. It's not necessarily just the HP, it may be Ian is not as affected by a smaller sized dose and may need more. HP is NOT well liked and I have found if I do not get directly down the back of the throat, they won't swallow and let it dribble out of their mouths. Deacon will let him go out the back corners where it soaks into his fur. So without knowing I REALLY go it down his throat, I can't confirm his doseage.
We are well versed in HP here having Deacon who has a sock (and fingers off of gloves and flip-flops and ... the list goes on) eating fetish.
I always keep 2 bottles - one opened and one unopened and sealed, just in case. I also have instructions taped to the cabinet (dark location) where the opened HP is stored with instructions on where to find the sealed bottle. I also keep a syringe rubber-banded to the HP bottle. We have this down to a science here in our household.
For Deacon who is just shy of 20 lbs and should be probably 19 lbs, he gets 10ml in a syringe; dose is supposed to be up to 1 ml per pound and they are supposed to be able to get 2 doses - emergency vet if they don't vomit in 15 - 20 minutes. I do in the bathroom and wait with him so I can ensure he vomits and I can see exactly what comes up. I straddle the back of Deacon holding his rough quite tightly so his head is back, tilted upward and his mouth is open. The syringe goes to the back of the throat, I tighten my grip and slowly expel - if I squirt it out quicker, not all makes it down. Deacon has an iron stomach so has always taken 2 of those smaller doses although I do give him a few minutes to see if it works on him. I swear Brodie almost vomits on his own if I show him the bottle... your mileage may vary.