Firstly, one flake of hay is not anywhere in the vicinity of enough hay. You want him on at least 2% of his IDEAL body weight in hay. If you want him at 600kg he needs 12kg of hay (as an example, without knowing his height and build I can't be certain of his ideal body weight).
Secondly, you need to feed by weight not volume. 2 litres of chaff tells me nothing. 2kgs tells me something. Invest in kitchen scales and weigh everything you feed him.
Thirdly, ditch the pollard. It's useless. And after some googling. Ditch the Easifeed Old Timer. It's sweet feed. It's the equivalent of feeding a child candy all it's life.
Now, if he has adequate hay, chaff really isn't a necessary ingredient either.
Look into either Speedi-beet or Hygain Micrbeet. They are beet pulp products that are a fantastic base for any horse's diet. On their own they will not add weight, but combined with the right things they are fantastic.
I would definitely keep him on a senior feed, but I would look in to Nutririce Veteran or Mitavite Gumnuts. Both awesome feeds!
Honestly, with adequate hay, a GOOD senior feed and perhaps some beet pulp I think you would notice a significant improvement.
As an aside, I completely agree with NdAppy that being a thoroughbred is no excuse for any horse to be underweight and nor is their age. It is rarely the number of years that an older horse has been alive that are the true cause of a lack of condition, rather it is underlying medical issues.