Hey I just happened to notice, no one mentioned that you still need to feed him if you do pen him part time. In case you don't know that. Unlike humans & dogs for eg, equine digestive systems are built for little, near constant amounts going through them. They don't cope well with periods of hunger. Empty stomachs cause ulcers & other issues. So if you lock him up, you will need to ensure a constant - or frequent small amounts - supply of low-cal hay. Native grass hay is generally far better than 'improved, cattle fattening' types such as rye grass. If you can't get low sugar, or you're not sure how rich it is, soak a little in a little water for an hour. The colour of the water will give you an idea how sugary - the darker it is, more sugar. Soaking & draining hay like this before feeding will leach out a lot of the sugar from over rich hay.
Well balanced nutrition is also important, with particular attention to providing enough magnesium. Esp if he is on hay, esp if it needs to be soaked, he will need appropriate supps to 'fill the gaps'. Magnesium is something which is really important, for metabolic health, among other stuff. It is not a fixed amount or ratio like many other nutrients, but more Mg is needed in times of stress, rich feed, overweight animals, etc. So something to learn more about.