He needs to eat less and get more active but first of all get him checked out by the vet to eliminate any physical or metabolic problems (thyroid etc). The vet should be able to give you good advice on getting his weight down as well. How long have you had him ? He might still be very wary of you (and hence showing aggression) because he doesn't yet trust you - that will simply take time.
Give him a high protein diet (look at the contents of the can or pouch), low in carbs (cats don't need carbohydrate) - two to four small portions a day (each portion being about the size of a mouse).
Don't feed him any treats except when rewarding him for good behaviour (using the cat scratcher instead of your furniture etc) and then only give him little bits of cooked meat or fish. Do all of this very gradually - he'd probably leave home if you put him on a crash diet !
Increase his activity levels by playing with him - toys stuffed with catnip (especially the ones that can be refilled) are great fun for a cat. My two thugs also love it when I attach a scrap of fabric and newspaper to a long piece of string and drag it behind me as I walk around the house - looks daft but they adore hunting this particular critter down....