Have you considered the NAF M Power which is very new to the market, its currently on offer too on here http://www.equestriansupplements.co.uk/power-900g-p-822.html
There is also More Muscle pellets by Equine America is also very popular and again, its not as expensive as the others but just...
I use NAF for my 3 boys (who live out in the summer months) and its currently on special offer here
http://www.equestriansupplements.co.uk/extra-effect-spray-750ml-repellent-horses-ponies-p-776.html
this one comes with a free 200ml travel size, perfect for shows or travelling :-)
I wouldnt recommend introducing any water (whether hosing or shampooing) unless you have the time or facilities to dry them completely. I would use a product such as silky manes by NAF or Absorbine Show Sheen sprayed onto legs every other day, as any made that finds its way on there simply...
I've got a 7yr old ex-racer myself, and I would recommend either NAF Superflex or if you are feeling a little flusher then Cortaflex HA - this really is the best joint supplement with proven clinical trials supporting it.
I would also recommend a very low protein, high fibre diet - with even...
We sell a product at work called ItchNoMore - I've not personally used it myself but it sells very very well in the summer months, perhaps you could give that a go? Its made by Equine America.... and is used by either watering down and using all over (every other day) or for concentrated topical...
There are two fantastic products on the market which sound like they could benefit you - Tye-Gard from Equine America, and Vitamin E & Selenium by NAF - might be worth giving them a go?
I would crush them up, and mix with a bit of apple puree or apple sauce - not too much though as of the sugar. The other thing thats worked for me is crushing again, but mix with apple juice and syringe - hope this helps! :-)
Equine America Buteless is a fantastic supplement, I would thoroughly recommend it. Its a competitive price too and the effects can be seen fairly quickly too :-)
Also, Im not a farrier - but I would have thought shoes would help support his feet? I can recommend an amazing farrier on Essex/Suffolk borders who has helped many a horse with navicular go on to resume a full and healthy career... i dont know what part of the UK you are in? PM me if you would...