R.M.
Hello Mama
I agree, you're probably trying to lay your child down when he is too tired. It's amazing how quickly kids change sleep patterns. You think you have a good rhythm going and a growth spurt will ruin it.
Watch for sleep cues, you'll know them, and try laying your son down right when he starts showing he may be tired soon. Even better, start a nap/sleep time ritual about minutes before he's tired, to cue him that it's time to sleep. These transition activities will be helpful when he's a toddler.
Lastly, I recommend to all parents I coach through sleep issues to find a CD of soothing music and start playing that during transition time and sleep time. If you ever travel, having music that cues bedtime will help greatly. Additionally, when he's older and wakes in the middle of the night, the music will help remind him it's sleeping time still.
Good Luck
R. Magby