aw babe, hugs!!!
15 mins seems like an eternity i know but i did it with my DD a few times and then she just learnt i wouldnt come in...if its an angry cry he probably is just peed off at you caus hes used to being rocked etc...
i might get crucified for saying this but its not in the gentle parenting section so i should be ok...if hes fed, not too hot or cold, no wind, etc, then its ok to let him cry (more like protest) for a while (and i know it breaks our hearts!!)

some ideas u may hav already thought of could be maybe hes sleeping too much during the day, maybe hes uncomfy (wind, teething), also around this age they get seperation anxiety
HTH hun