Whats my best option

Hi all. A little over a year ago we purchased a betta from our LFS and about 6 months ago he started to develop a white lump by his back fin. I asked the LFS if it's of any concern and they suggested I keep an eye on it as bettas are prone to these. However, the past month it has gotten massive. He just sits near the top to stay propped up now or I will oftenl also find him laying sideways near the bottom of the tank. He will occasionally go for a swim, and he still eats like he's never been fed a day in his life. As long as he's acting somewhat normal besides the resting (I could only imagine how annoying it is) should I keep giving him the best life I can, or would you recommend euthanasia? I don't want to do that obviously and my daughter would be devastated, but I don't want the little guy to sit and suffer for our entertainment. Here is a picture while feeding this morning...Also all that's in there are 3 nerite snails as well until I can move them to my community tank. Parameters are Ammonia- 0 Nitrite - 0 Nitrates - 10 and ph of 8.0...Thank you in advance for the suggestions.