Mr Eric is now in rabbit middle to old age (they tend to live to about 7-8 years old on average). The castration will improve life expectancy. Is he a typical brown rabbit (watership down and wild type?)
Has he been wormed recently? My two girls lost a little weight when they had worms (they can get a bout even if indoors and no other animals, the nasties can lay in wait for a long time) and you can get some panacur from the vets or online which is great stuff for getting rid of them and rabbits seem to love the taste of it!
Since his teeth outwardly appear good and still a good appetite I doubt there are any gum problems, but just check for any inflamation arount the roots of the teeth incase. There are higher fibre foods that can be purchased to help him put weight on if it is just through his age that he is losing - burgess do a good range of them. It may be worth a diabetes blood test at the vets as well, as it can strike in middle age with rabbits
However, some rabbits as they get older do just lose weight in winter then put it back on in summer so hopefully it's nothing to worry about.
Hope Mr Eric gets better soon and keep us updated xx