Looks like you've done well with him then. He's still a little underweight, definitely undermuscled. So if he's still gaining any at all, I'd consider leaving the diet alone or possibly even cutting down slightly.
If you haven't done so, get his teeth done & get him wormed *effectively*. I'd also consider treating him for ulcers or at least adding a probiotic to his feed & I'd consider getting the vet to check for other issues - take bloods, nutrition, metabolic, etc.
He is on green grass all day and he gets grain and alfalfa cubes. I am about to start him on some type of oil. Haven't been able to figure out which brand(some type of fish oil).
Some considerations... Depending on what sort of grass, etc, it might be that it's too much for him. Grass can be high in sugars & lead to metabolic & gut problems. If it's lush & not very stemmy, maybe he could do with some stemmy hay or such too. What sort of grain does he get & how much? Grain is not necessarily bad, but it is high starch & not easily digestible for horses, which means it's often not the best option where more energy is required. I would probably opt for a healthier low starch/sugar, high fibre alternative such as beet pulp. Especially if you can't feed the grain/starchy feed little & often over at least a few small meals daily.
Alfalfa/lucerne is a great horse feed generally. It's high in energy & low in sugar. Stemmy stuff(1st cut) is also extra high fibre. It's high in protein, calcium & many other nutrients. So it's usually too high in energy for 'easy keepers', the high protein can be a prob for some horses, particularly aged, and it should be fed as part of a balanced diet to ensure a good Ca/Phos balance & such.
So re diet, I'd consider changing the grain to a healthier alternative, do a diet analysis(I like online feedxl.com) to work out a balanced diet and consider feeding more roughage. Horses can fail to thrive due to imbalanced nutrition rather than insufficient calories too. If you're going to feed oil, start with tiny quantities & feed it little & often. He won't need much. Especially if you're going to feed grain or other high omega 6 food, I'd definitely avoid corn oil & go with something high in omega 3 such as fresh ground flax/linseed. I wouldn't be inclined to feed fish oil to a horse though.