He has Burgess Jr & Dward pellets - 1 handful - this is 17% crude fibre and 16% protein. I would consider decreasing the amount of pellets they have slowly and also consider changing on to a higher fibre and lower protein food. Science Selective which include 19% fibre
http://www.supremepetfoods.com/product/science-selective-rabbit/ or Bunny Basics
https://www.thehayexperts.co.uk/Foo.../Bunny+Basics+Adult+(%252FT)+:+4.5kg.245.html are both high fibre foods. You would need to change over very slowly over a period of about 2 weeks.
A little srping greens in a morning - should be fine
A basil leaf when we eat dinner. - should be fine
One yoghurt drop at bed time and a quarter carrot too - would suggest not feeding yogurt drops as this is not part of their normal diet in the wild, rabbits are also lactose intolerant. Carrots are high in sugar and may be worth avoiding until you can sort out the sticky botton.
Obviously he has fresh hay all the time - he should be eating roughly the same size of his body in hay a day.
He's had the same diet since we got him - as rabbits get older there needs change,
He rarely eats fruit cos of the sugar content. - all high sugar and carbohydrate foods should be avoided.
He sometimes has a couple of carrot crunchie treats (the natural range from P@H) The contents of these carrot crunchies are Green Carrot Shape: Wheat (min. 23.9%), Maize (min. 13.9%), Wheat Feed (min. 9.2%), Ground Peas (min. 1.6%), Sunflower Oil, Mixed Herbs (min. 0.2%) (Oregano, Marjarom, Thyme, Parsley).
Orange Carrot Shape: Wheat (min. 25.8%), Maize (min. 13.9%), Wheat Feed (min. 9.2%), Sunflower Oil, Carrot (min. 0.02%). Coloured with EC Additives. I would avoid these as they are high in carbohydrate and proteins which all can cause dirty bottoms. Some of the ingredients are not ones which would naturally be found in a wild rabbits diet such as sunflower oil. You can supplement these with dried leaves and flowers such as
https://www.thehayexperts.co.uk/Natural+Treats+%2526+Herbs.7/Natures+Salad.295.html or
http://forums.rabbitrehome.org.uk/showthread.php?t=296573 these are far more natural and beneficial for rabbits. Collecting your own leaves and drying them may also help your bunny keep clean
The other things to consider is if he is overweight and cannot reach his bottom to keep himself clean, if he is you need to consider cutting out the high sugar foods, treats and reduce the pellets. If he is elderly and has arthritis he may not be able to reach his botton, a vet review and may be an antiinflamatory may help him if this is a problems.
Dental buns often also are more prone to dirty bottoms, this can be checked out by a vet.
There are a few gut problems which could be causeing problems which you would need to speak to a rabbit savy vet, however start witk h the easy option by changing his diet as this may well sort out the problem.
It is good that you are trying to look at what you can do to help you bun.
