I've just re-read through this and this poor fellow has been on 5 different kibbles plus chicken and rice in 3 weeks!!!! (Beta, Royal Canin, Arden Grange, Wainwrights, and JW)
Please just stick to ONE food until he gets better, and speak to your vet tomorrow fro advice.
He may well have got an infection from the horse much but you said his poos had cleared up, so I'd be more likely to think it's the diet change that's causing the problem.
Young puppies should never be changed as rapidly as this, they are babies. No offence intended, just trying to help.