I"m a firm believer in laying down with an infant/child when trying to get them to take a nap. I think this helps incredibly with bonding, as well as give the child assurance of a parent being close by.
That said, IMO, your child is still going through a transitional stage....and I think this is normal behavior to some degree.
I'm wondering what time you nap during the day? If your child is napping later in the day....it makes it harder to be truly sleepy for nighttime, KWIM? In our home, unless there are rare circumstances.....we won't lay down for a nap after 3pm. Later than that, tends to be too close to the bedtime of 8-9pm.
An 18month old needs to be active and running around for part of the day..
Does your child get enough exercise during the day?
And....if this child was in the same foster home for several months.....this could still be an issue. Good fp's or not, the routine was with them....and your child may still be trying to figure out why things aren't as they were before.
Sounds as though he's beginning to bond, though...because he's not so willing to go to strangers. THIS is a really good thing (but I think you already know this). Just keep trying. Keep holding him whenever you can. Lay down with him during naps---and both of you can rest. Spend time with him and be patient.
Stick to routine as much as you can; show him you're there....I suspect things will improve in short time. You're definately on the right track.
Sincerely,
Linny