As long as he has a bottle to settle himself back to sleep he won't be happy during nap time because he is not learning to do it himself. In theory that is what he is doing now - the nap, wake, settle self by crying self back to sleep, sleep. It should get better for you eventually but the parents taking the bottle would certainly help. Sometimes using a soother for naptime if they are used to the bottle at home can help some kids.

Both of my little ones - 13 m and 14 m are both still waking through the night - one is being nursed back to sleep and the other one is being taken into parents bed and given a bottle and allowed to stay there. Not helping with naptime here but it is getting better for both of them.