That's okay, if you end up needing to use the fibreplex you can put some warm water with the 1tbsp of his nuggets/pellets and create a sort of paste.
Then squish the 1ml of Fibreplex into the same bowl and see if your boy will eat it on his own, if not you can cut the end off of a 1ml medication syringe and administer it as you would his medication/support feed.
These are supposed to be tips to give fibreplex, as it by itself has a bad taste meaning your pig will be unlikely to want to take it. I understand you do not have critical care and it is not technically needed if your boy is not losing weight.
There is a section in this guide about cutting 1ml syringes and making pellet paste -
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures