I would first, make SURE that you are producing enough milk.
And make sure that he is latching on properly.
If the answer to these 2 questions is no...then see a Lactation specialist.
Now, are you nursing him, on-demand... or by a schedule????
Babies, need to feed, on-demand.
IF babies are fed according to a schedule... they do not get enough intake.
My kids as babies, nursed an awful lot more, than only 5-6 times daily.
And, at 3 months, that was a growth-spurt period... and during growth spurts, babies NEED to feed more frequently and they need more.
Supplementing with Formula, means, that AFTER you nurse, you give your baby a little, Formula. BUT if you replace each breastfeeding with Formula... THEN your milk production, will lessen.
And, be aware that some babies will NOT take a bottle.
AND some babies, if they are given a bottle or Formula in a bottle, then they might not... want to nurse anymore. Because bottle feeding is only by gravity and it is easier to suck from. That happened to my friend.
You need to make sure you are producing enough milk, and that your baby is latching on properly, and that you are nursing on-demand day and night 24/7, and per their development and growth-spurt needs, it is normal and common... and a baby will NEED to nurse, even every single hour. That is called "cluster feeding." For example.
My kids as babies, when they were under 6 months old and just infants... nursed every 2-3 hours. Sometimes every hour when they were growing. They had ginormous appetites and I nursed them on-demand. And they grew like weeds. I used both breasts each nursing session.
Nurse your baby, more often.