I'm very concerned that this person is apparently frequently sending out joeys that are only 6 weeks OOP. They need to be with their mother for at least 8 weeks. You were lucky that your two females came to you bonded, and that is the reason you did not notice the troubles that your new fellow is having.
Not only is he stressed having been taken away too soon (any age glider is stressed when moved to a new environment) but, he will need extra care from you in order to stay warm, as you know, and ensure that he is able to eat on his own. At 6 weeks he should have been starting to eat the food that his parents ate, depending upon what sort of diet they offered.
This site
http://www.suzsugargliders.com/helpforrejectedjoeys.htm
is an excellent resource for taking care of joeys, whether rejected immediately or if taken young, as yours has been.
He isn't bonded to your other females and you've skipped the quarantine and introduction process without noticing (I'm presuming, as you said you put him in a pouch with them successfully during the day) any initial aggression toward him. That doesn't mean that they will help take care of him and yes, you will need to be the one to keep him warm and well fed. Take a look at that site and feel free to ask as many questions as you have. There are wonderful and knowledgeable folks here that will be able to help your little guy get through this.