what was he getting before?
The generic "scoop" that basically everyone has holds right at 3 lbs. Of grain...so 2 scoops would be 6 lbs.
4 handfuls of alfalfa cubes is kind of hard to guesstimate, but I would put that at about 2-2.5 lbs.
2 flakes of hay...well that's 2 flakes of hay :) (and weight can depend on each individual bale and how it was cured/baled)
If he's average size, let's say 1000 lbs, and on a basic sweet feed (at 11% protein and 3.5% fat), he would need between 6-8 lbs of grain per day, coupled with roughly 11-18 lbs of forage (grass, alfalfa, and hay combined). - this also depends on the individual horse (for example, people with high matabolisms can eat and eat and not gain a pound, while people with slow matabolisms can eat just a little bit and either gain or maintain their weight)
For comparison, I have a 26 yo QH/TB that is on 12 lbs of senior per day (3 feedings) - lush pasture - and 6-8 lbs of soaked alfalfa plus one small flake of grass hay every day...and that's his maintenance point :)
I would say that so long as he's maintaining his weight, and not ill-affected by the extra starch and sugar from the sweet feed that he should be just fine. There are many people around here (GA) that feed sweet feed only (no pellets) - - and swear by it. It just depends on your horse :)