I think the amount he is taking now in 24 hours is fine, but obviously you don't want it to drop any further. We had the same probplem with DD but we found with her that solids actually helped her intake. We had to drop to 3 bottles (plus the NF) just before 8 months to keep her intake up.
I think really, you will need to take it slowly and see how he does - make sure you keep milk the priority and solids an hour or so after milk feeds. You could try reducing one of the night bottles if they are a regular thing to start with.
Laura