That really seems very little! Especially split into 2 feeds. Give him more- I couldn't say how much because I feed straight oats, but PF got 3 scoops a day in light work.
Also feed ad lib hay if possible.
Without having the actual weight of the feed it's hard to tell but that doesn't sound very much feed to me.
The list for weight gain is get teeth checked, make sure you have covered all worm types in your regime (better still get a count done) then look at your feed.
Are you giving ad lib hay or haylage? If you provide enough he will learn to self limit no matter how much you give him.
I would add a generous amount of oil to his feed for starters, and feed pink powders for at least three weeks.
I don't know what fibenergy is but am assuming it's like alfaA?
I would ring Saracen and ask their advice.
We struggled with a horse last year who field walks and was losing weight , they advised a diet and within a couple of weeks he looked better. Now he simply looks stunning, shiny glossy coat, rippling muscles etc.
We feed him on Prep 14 plus their Equi-jewel, a form of rice bran.
My only complaint is their feed is now silly money , but as it works better than anything else we tried, we stay with it!
In comparison the horse above is getting a heaped scoop (round handled scoop) of Prep 14, a handful of equijewel, heaped scoop of Alfa A, joint supplement plus oil and speedibeet. This is twice a day excluding the supplements in the morning feed. he is 16 hh lightweight and eats two big haynets of haylage every night.
He isn't in much work but refuses to eat much during the day when out, he only nibbles occasionally and dreams the rest of the time (about sex probably, he's a stallion
a friend has a 14h show pony,shes struggled to get weight on the pony,she tried different feeds,the pony looked like a racer,she was told about simple sytem feeds the difference is amazing, www.simplesystem.co.uk