Just so you know, the nutrition content is pretty much the same as normal adult pellets, so if he has those at the moment the Fibafirst would be fine as his main pellets. But as they are so much lighter than normal pellets it is easy to not feed enough, so I'd weigh them so you know how many to feed. A handful for me is around 30-40g.
My boy Barney has bad weight-loss issues and he does well on them because the high fibre levels help his gut stay healthy, which then has the knock-on effect of his gut being able to absorb nutrients a bit better. I can also feed more of them without the weightloss that he usually has if I feed a lot of pellets - I know that sounds bizarre but more calories doesnt equal weight-gain for him because of the effect higher protein/higher fat, lower fibre foods have on the health of his gut. High nutrient foods don't work long-term for him, they make him worse.
Just thought if mention it incase it could help Mack.