Is he in the cot for his day sleeps? Can you move the cot right up against your bed and drop the side, ensuring the cot is securely against your bed? ie at arms reach. I know with my DD if she is not fully/deeply asleep she wakes up when I put her in her cot, if she is really asleep she doesn't wake up. Perhaps he is not in a really deep sleep? Try covering the cot mattress with something of yours that smells like you ie something you have worn, a rug you have been lying on? Has he got a comforter with him ie a cot teddy, a ruggie etc? Just some ideas...