'I don,t under any circumstances expect the playdate to be reciprocated and I always pick up her ds from the school and always drop him back off I do not expect her to run around when I have given the invite.'
That is very good of you but does she know this? In any case she may FEEL obligated.
'I would be delighted if somebody was inviting ds around every week.'
But just because you would, doesn't mean others would, unfortunately and that seems the problem here. I am sure she does appreciate the invite but it just seems too much/too different to what she does/feels is right. There will be other mums who are more enthusiastic about regular playdates so maybe widen your circle-there are probably mums who would love weekly playdates amongst the class or from the boys (or girls) at cubs/other after school clubs.
'But then again I realise alot of people already have a ready set of play mates in the form of siblings so don,t feel the need.'
This is true in my case and I am lucky there (however some friends say their kids never play together and argue all the time) but they still do see friends and have friends round too but it is more occasional. Siblings can feel excluded/play up etc. and ,with 5 already, it can be chaos.
'It can be quite lsolating at times being the parent of an only.'
I do feel for you here but think maybe joining clubs etc would be better? That way you have a regular way of him meeting friends without all the awkwardness. My DD's best friend is an only and her mum invites DD alot more than I invite hers but it isn't overwhelming; perhaps once or twice a month and she alternates my DD with 3 or 4 other friends over the month so that she has different ones each week. I did feel awkward that she did it far more but the mum reassured me that she didn't expect it at all and knew it was harder for me etc.
Perhaps you could have a word saying sorry if it got abit much (maybe say your son asked the friend who told him) but your son really likes him coming to play and you understand it was abit much for her and are happy to do it abit less? Comprimise could be the key. Good luck!