An owner occupied house is deemed in housing law not only to be a personal possession - it is a resource for future generations. If it is not looked after properly it can cause 'blight' - a legal term - to adjacent, attached houses and to a neighbourhood. Make an appointment to see a qualified housing officer (must be qualified! don't settle for an admin worker) in your nearest housing advice centre. List concerns and take pictures. The ivy etc. could possibly cause damp in your house eventually, and the roots of the trees in his garden could possibly grow to disturb and damage the foundations of your property. The Environmental Health department can investigate whether his living conditions are likely to encourage pests/rodents and impact on your/ your neighbours' health.
I would try to make friendly personal contact in the first instance to see if there is anything you can do. It may be he is on a very low income and/or disabled mentally or physically, in which case, he may be able to get financial/practical help towards repairs and improvements(again, the housing department of your local council will tell you/him what is available). If this is impossible, you could raise concerns with social services and local voluntary agencies about your neighbour 'not coping'. There are voluntary organisations in most areas to help and support people on their own over 50 who have mental or physical health problems or who are just plain lonely; and others which provide volunteers to assist with housework and gardening. Housing and social services are now expected to work more closely together. Hope this helps. Good luck.