For weight gain feeding 3 times a day is ideal but feeding morning and night would be ok if you can't go three times a day. The dark is not a problem to horses.
You probably already know this but when you first get him, feed only a tiny bit at each meal because he will be stressed from moving and suddenly getting heaps of food will not help his smooth transition! Build up the quantity over 10days-2weeks until he is on the amount you want him to be on then maintain this quantity until he's at a healthy weight. Remember when it comes to spring, the grass will be lusher so he probably wont need so much high sugar/starch food, but fiber and micro-nutrience are still needed.
I would also suggest giving hay for most of the year. In winter, grass isn't growing so there is pretty much 0 nutrience in it so hay is needed (Grass only grows when it is consistently above 5degrees C). In spring- autumn I still offer my tb hay because it provides a lower sugar forage option for her. Seen as she is eating the hay atm she probably wants it so I will contiune to provide it, if you follow my logic...
As for flexibility, I like routein and so do horses but I also feel it is important that a horse can cope with change. Alli gets fed between 6.30 and 7.30 mon-sat but on sunday I might give her a small amount before our ride and the rest after. She thinks its great- 2 breakfasts!!! In the evenings she can be fed any time from 4.30-7 depending on whether I had school or work, or if I work her that night and for how long I work her. I think its good for her to be used to a slightly flexible routein- what if I have to go away and the person left lookign after her has to feed her at 8am and 7pm?
I want Alli to be able to cope with slight changes as it stops her getting as stressed if there is ever a change. She is prone to ulcers and I have found slight flexibility is better than ridgid structure which can be upset (thus upsetting the horse). Life is unpredictable so there needs to be some form of give in a horse routein.
Oh, and have fun with your new best boy!