If you are mixing it with water, this could make it too bland. Put a little breast milk into a tiny bowl and add only a small amount of the cereal (it should be runny until baby gets used to eating out of a spoon). Use immediately because something in the breast milk makes it start to get watery.
Be sure he is hungry but not so hungry that he gets impatient. Allow him to smell the familiar smell of "you" in the milk before offering it to him on the spoon (the sight of the spoon may even bring back the memory of previous feedings so hide it at first). You may even want to see if he grabs for it. Maybe even us a clean hand with small amount on your finger and allow him to suck it off.
Sometimes babies feel that mom is only supposed to nurse. If your husband is willing, see if your little guy will eat for him instead until he gets used to eating and then you can try again. Be sure you are not too close; sometimes babies can even smell mom and think "nurse". (My son could smell milk from across the room and want to nurse.)
At 7 months, a baby's taste buds are developing. Maybe you should skip the cereal. (Have you tasted it? It's awful!) Veggies are a great start (not too sweet like fruit; not too bland like cereal). If veggies work, you can add the cereal later as a thickener to veggies and/or fruits.
I hope this helps.
P. <><