The jars are usually on offer, and are great as a stand buy but I know what you mean about them beig more expensive than homecooked stuff - not as nutritious either!
As lulu says at this age babies dont need 3 meals a day, they still get most of their nutrition from milk.
As a rough guide,
7am - milk (5 oz)
8.30 - porridge with some fruit or plain as he doesnt mind it, a small cup full maybe?
12pm - lunch - again small cup full of whatever, or a jar, sometimes a yoghurt afterwards.
Tea, normally doesnt have very much, yoghurt and a banana or half a slice of toast
6pm Porridge before bed,
6.30 Milk (5oz)
He usually has a full bottle throughout the day, snacking as and when? Also his cereal is made with milk too as its a sneaky way of getting it into him as he isnt hat fussed on milk. Also rice cakes, and other snacky things
I might be wrong but I wouldnt worry about him eating too much - I dont think he would eat if he wasnt hungry? Lulu will probabaly be able to answer that with professional knowledge though!
At this age they are learning all sorts of new skills - sitting up, crawling, shouting so they are using loads of energy which is why he's eating loads!