As others have said- I wouldn't free feed. Puppies will eat, and eat, and eat some more. They don't just stop when they are full. Granted, they should burn the calories a lot faster. Best practice is to use specified meal times. Read the feeding instructions on your bag for a puppy his age, and divide it up into about 3-4 meals a day

Bullies easily pack on the pounds- we are currently working on helping our 2.5 year old Effie to lose some weight. It's not as easy to lose once they've put it on! (Her issue was my husband giving WAY to much people foods lol). With their weight issues, and heart condition you mentioned it would be best to track how much food you actually are feeding on a daily basis, in case issues arise in the future.
Good luck! You're baby is beautiful.