Congratulations Kazza! :) Our DS was placed with us when he was 4, now 5.
Plenty of 'rough and tumble' worked for me when trying to get closer to him (he was very distant with me, did not want to get close), he especially loved to be on my back, constantly! so we played lots of horsie horsie games etc! He also liked sitting on my lap, we played 'guess the letter' games etc on his back. Basically he always had to be playing back then!
Games like cars and building train sets were popular in our house, as well as cooking/baking and the park, he always wanted me to be involved in his play, and still does, so I am forever climbing up the climbing frames/swinging on the monkey bars etc (I must look like a mad woman!), he also loved dancing with me (jumping around!) to the soundtracks of his favorite TV shows.
DS was fine with DH from the start, it took a long time for him to build up that confidence with me.
I suppose it all just depends on the child and their interests at that age, DS does not like sports (football etc), it almost terrifies him the thought of taking part in those kinds of activities, but he loves to go out riding on his bike etc and is very sociable with children and adults. Try to focus on a few things they enjoy I'd say and start from there.
I am not sure if any of that is any use at all to you! But once your boys are home please just message if anything crops up as we've had a years worth of experience with our boy that age :)