Hi Missy if your pony is over weight he is eating more than he needs.
A ponys gut is designed to turn little into fat and needs plenty of fiber for the gut to do its job properly.
I would suggest you limit the amount of grass he has and if he is not working dont feed him as well, or feed a high fiber diet.

Ponies fare better on grazing that is not rich or lush as many pony owners put ponies into feilds that are topped and rested this leads to good grass but this is not good for pony just left to graze and not much work.

The sugar and protein content is some grass is very high this makes pony seem naughty and spooky if was not prior to good grass.

If pony is on good grass plus being fed cerials this will create a weight problem and behaviour problems as pony is full of sugar abit like a child that drinks sugary drinks and eats to many sweets the behaviour changes they are hyper active and unpridictable.

I would first look at limiting ponys grass or strip grazing or keeping pony in on some low protein hay or hay and straw mixed but allow pony out to graze for shorter periods, whilst finding gentle exersize to start like walking out and some up hill work in walk leading if need be for short periods.
If you are also feeding pony change from cereal to high fiber diet slowly over 10 days.

Ponies that over weight stuggle to move correctly and they can strain joints and heart and lungs along with the risk of laminitis.

You will find as the sugar level starts to return to normal levels his behaviour will change and as he loses the weight he will find it easier to move and and stay motivated.

Exersize needs to be steady at first as his heart and lungs will have to work harder plenty of walking and some slow hill work just walking will help.

As i do not know your ponys routein and weather you are feeding as well as him grazing or if he is working or not but being over weight is not healthy for pony and may lead to problems.

I hope this may help you.