Avoid the canned ones as they sometimes contain salt as preservative.
The frozen ones are great, you can either defrost one in hot water or cook it.
I find putting one pea into a cup with some hot water for a few minutes will generally defrost and cook it out a little, then cool it, shell it, seperate the two halves and then cut one halve into smaller pieces, feed one small piece at a time. He doesn't have to eat it all! Remember to remove any uneaten pieces that sink to the bottom and are not eaten after a few minutes or it may pollute the water.
The tank looks great, I couldn't see from the picture, but make sure you have a fitted lid or cover as Bettas can jump, although I'm sure your boy won't be doing any of that until he feels much better.
I hope he gets better soon