I agree with PP about him learning to self soothe. Plus, he's had 4.5months of waking during the night, he is used to waking and being helped back to sleep (feed). It should settle down soon. If it doesn't you may want to try dropping the df completely but give it a while first to see how he does on his new routine of df at 10.30pm.
Don't forget he is only 4.5months. DD was this age when she started to sleep through but a lot will still be waking to feed at 9months or more so hes doing well