I wouldn’t use a ball that has treats inside, it’s a very quick way to teach a dog to chase! That’s the last thing you want with a working cocker.....
At this age he should be sleeping around 18 hours a day. Don’t worry about “mental stimulation” just yet, that’s needed once he’s around 9 months old. Have short training sessions, 2-5 minutes of sit, or come here (That’s what is really meant by mental stimulation) use the time now to bond with him and get him into a routine. Don’t call him to you if you’ve asked him to sit - always go back to him and praise him until he’s much older! 2 years on our youngest still looks to go back to bed after morning food / exercise and sleeps most of the morning, useful now I’m working from home!
The next phase for you is the bite inhibition stage - you may want to read up on “cockerdiles” on here.
Welcome to COL
Jayne