His head has been like that for a year? Weird. Some bettas with the marble gene can go from a dark color to white but i don't think they would do that so late in life. Try messaging oldfishlady or sakura8. If anyone knew, it would be one of them.
have you tried a general tropical fish forum? If not, try tropicalfishkeeping.com That is the parent site of this one. Your ID for here works on there as well.
I would copy and paste what you said on here - to this forum
I only own bettas so i seldom go over there but someone might be able to tell you what's up with him.
3 years is a pretty good run for a betta. maybe just old age?