Food won't fill him up more than formula, it will actually displace calories of formula at this age and probably result in him getting less all up. It is very early to consider starting solids and I would suggest doing some research to determine whether you are actually comfortable with the risks. A baby gets their main intake from breastmilk or formula until they are one, food is purely to introduce more nutrients, flavours and textures which wouldn't be neccesary at 3 months.

At any rate food should be offered AFTER a milk feed.