If (as it sounds) he quite a bad dose of fin rot - his tail may not repair 100% ever - so if you have treated him according to the directions - I would say to not continue dosing - hard I know but most of the bottled meds are actually slightly toxic to the fish - just more so to the problem your trying to fix - a bit like radiotherapy.
If he seems happy enough and is eating etc - just do what your doing and keep on top of keeping the water good - in time, his tail will continue to heal but as I said - it may not look 100% how it did before - and some bettas even when theyve never been 'ill' look like theyve been shredded - its just the growth pattern of the indivdual.
As long as you feed him and his water is good - he'll be fine - just watch closely for a couple of weeks - if you wanted to err on the side of caution - id suggest (yes yes - i do have shares
) a tiny dose of melafix and pimafix (herbal treatments - I never got my fish to smoke canabis so this is as good as it gets
) every other day (eg a quater of the labelled dose)
Its enough to stop a fresh outbreak of finrot or secondary fungus infection without being hard on the fish or filters etc - and maybe just a slight boost to the fishes self repair kit
But nature n time/patience are probably the best remedies
BTW _ I seem to remember you asking about tanks for around £50
well - this price on this one is more but Pets at home are much cheaper - I got my tank (same as this one for my bettas etc) for £45 from P@H
http://www.charterhouse-aquatics.co.uk/catalog/arcadia-tank-aquarium-p-1588.html