Awe, Poor little guy.He has really pretty colors. It does look like tail-biting to me too. You can tell by the u shape "cut outs". Tail-biting happens when a betta is either overly stressed or bored...Sometimes though they just bite to be biting, I think.I have 2 bettas who bite themselves frequently..I have honestly never seen their tails full grown.
If hes bored, you can try moving some of this decorations around...try live and frozen foods to help with regrowth..
If stressed I would keep his light off and make sure the filter is well baffled...You could also try adding a few dried oak leaves or Indian almond leaves if you have them.
I have read sometimes soaking the food in garlic juice helps to encourage the appetite.