The way you describe him sounds like he is perfect. You should be able to feel his ribs and he should have a waistline, as you described.
Here is a good guide:
K9 Station Dog Training & AuxArcs Australian Shepherds
If he is hungry you can add things to his food that are low in calories, such as green beans.
Mikko (USDAA) MAD, SAM, JM, GM, RM, SS; (NADAC) EAC, EJC, TN-E, TG-E, WV-O, OCC, HP-O, VerO; (AKC) NAJ, CGC; NW1 - 8 year old GSD