When I first started breeding bettas, I always pulled the male as soon as the fry became free swimming. Almost every single time, the male became "mopey" or depressed. After reading an article somewhere by OFL, I decided to give it a try to leave the male in with the fry as long as possible.
I have lost count of how many spawns I have had over the years. I have yet to have the male eat ALL the fry. I have found if they are in a small container, or if I feed too sparingly, some of the fry will disappear, but never all. I have since learned to give a plenty big enough tank and feeding things that will live in the tank for a while lends itself better to genously feeding without the worry of spoiling the water. Lots of leafy plants (wisteria is my favorite) allows the fry to swim away from the nest, without Dad getting too anxious and over worked trying to collect them all. After a few days, you can start to feed the fry a little further and further away from the nest, and Dad will give up the nest. Once he has given up the nest, you will wittness him "calling" the fry when you feed. It makes it easier to switch to non-live foods, and bring more variety to their diets. Dad will let them know what if meant to be eaten. At some point, Dad will stop surveying the tank continuously, and start to stay in one small area. That is when I pull the Dad. He pretty much has left them to their own, and is sort of fed up with their shenanigans.lol Also, when they have first left the nest, Dad will still continuously swim the tank. As he passes by the ry, they will swim after him. Of course, they cannot keep up, but they get way more exercise and are much stronger with Dad's presence.
All that being said, you do have to watch to make sure Dad does eat, and does rest. Plants make that easier for him as well. I usually have them bundled, then when the fry are first free swimming, I take them out of the bundles and spread them through the tank, making sure some are just a few inches below the nest for Dad to rest on.
Deciding to try leaving the Dad with the fry was one of the best things I decided to do with breeding bettas. It is so fascinating to watch the interaction and behaviors. I encourage every one to give it a try. Don't jump to the conclusion that he is eating them when you see him grab one up. Watch him for as long as it takes to see him open his mouth again. Several times over the years, I saw a Dad grab one and swim to another part of the tank and release it. I always wonder what I missed. Was their a sibling rivalry getting ready to break out? Was he reminding them who is in charge? What was the purpose of that? lol Guess I will never know unless I learn to speak bubble. lol
If you do pull Dad, and he is depressed, show him a lady for just a few minutes, especially if he has a favorite. or maybe even give him just a few fry to look after. One more tip, if you do leave Dad with fry, don't let them see a female for they may decide they would rather spawn again. I don't know if this is true or not, for I always keep all other bettas out of sight of the spawn tanks. Oh how I love these fish!