My NF pony came back of loan with laminitis, she had never had it before. She was only 14hands and on a weigh tape was 450kgs, abour 100kgs over weight.
I fed her 1.5%of her body weight in soaked straw/hay/and/chaff with a feed balancer split into four feeds. In the acute stage she was on box rest.
It took ages for her to loose weight. Over winter all she had to eat was straw and feed balancer and was not rugged. By spring she was her target weight and with strip grazing have managed to keep it off.
I know someone has advised lunging but be cautious as he is very overweight. I would suggest lunging or long reining on an oval at a walk to start off with at a 'nelly the elephant' pace.
To weigh hay I'd get a spring balance. Cheap and easy to use! Even stuffing nets it is really hard to be consistent with what you are feeding. Given that he really must loose quite a bit of weight I would def get a balance and be very strict with yourself when weighing! http://www.mptools.co.uk/products.asp?partno=250380 the 12.5kg one should do you nicely and is under £2!