My best advice is to not make a big deal when another child comes over and just "ignore" your son going up to his room. If you say things like "Oh look who's here" or "why don't you two go play..." it may make your son feel pushed or pressured which will eventually make him feel self concious. If you ignore it and try to give the visiting child something to do to occupy themselves, then maybe eventually (days, weeks, months later) your son will eventually be curious and want to play too. Your story made me remember how I felt when I was little. I wasn't shy or unwilling to play with guests but I felt VERY self concious when my mom watched me play or even subtlilly pushed a friendship. I just imagine your son maybe going and playing next to that child and then eventually interacting with them if you are not the least bit involved.

Also, I remember reading something when my daughter was about that age that 3-4 year olds will play side by side but not really engage eachother until a year or so later. Something tells me it will all fall in place for you and your son.

Thanks for your advice mariene, I will try and let him be when friends are over and see how he responds. I am aware there is a long process ahead of my son with his socialisation, I just hope he makes good friends along the way. Cheers.