toddler anxiety

Hi,
I am after advise on how to get my 17mth old to be more comfortable around others.

I am a sahm of three, this is my middle child and I have never seen a child so clingy before.

Every week we go to mothers group with the same ladies and same kids and have been so for over a year now and he will still cry if I am out of sight and none of the ladies are able to calm him down.

I tried testing him in a crèche and he lasted 30mins before someone had to come get me as he was inconsolable., he calmed as soon as I held him.

I went to a daycare and sat with him for an hour to see if he might feel comfortable after a while and he didn't stray even 2 feet away from me the whole time and as soon as I stood up he ran back to me.

At home he follows me from room to room the whole day long. And he is only comfortable with myself, my husband our eldest and my mum. Everyone else, even family he sees on a regular basis he is still weary of.
He cries if some people (including my brother) come to close to him at times and if I am holding him and they walk near him he just stares and moves as far away in my arms as possible.

I never had this problem with my eldest so I am unsure of how to tackle it and since his time in the crèche he is waking repeatedly at night and only calming if we stay with him which is not possible to do every night.

I don't want to do anything to drastic to fast as he will only go backwards but I don't want him to be scared of everyone especially his own extended family.

Please help.
Thanks