I think it depends on why you're on the diet. If you're going elemental because of being underweight, then a bit of occasional juice etc *might* be ok. If you're doing it to rest the gut, then my understanding is that it's best to stick to enteral feed alone.
You also need to remember that the feeds have been formulated for exclusive feeding, so if you go adding juices etc, you might disrupt his nutrition and even overload him on some vitamins, which can be harmful.
That is my opinion, but I think it would be best if you waited for Writer to come, as she really is our resident expert
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Ivy.
ps. While you're waiting, you might like to look at the "nutrition" link in my sig.