If your horse is in good weight now, keep doing what you're doing basically
It is fine for a horse to lose a bit of condition over winter, but if you find he's dropping off, you could feed a bit more, esp hay. No need to change to different ingredients necessarily though and I wouldn't choose sweet feed and other grain as a rule. That is high energy type feed for hard working horses, and high sugar/starch too, which can cause issues, esp if you can only feed a couple of meals daily - little & often is best & even more important when you're feeding 'high GI'.
For some good info & further sources on feeding horses, there is feedxl.com safergrass.org & Dr Kellon's site, among other good ones to learn more from.
So, my opinion is that I disagree with your farrier on the feeding note. On growing, yes, horses don't finish maturing until around 7yo & depending on the breed, that can include growing. Clydies & the likes can be still growing at 9yo! One reason I don't like starting & working babies & youngsters...
Oh, nearly forgot to mention, I'd also be giving him a good quality nutritional supp to ensure he's getting well balanced nutrition. Feedxl.com is one program you can use to work out what your horse needs.