Find Living Relatives

Nobody mentioned adoption.

The man looking for his father was fostered, not adopted, and he knows the names of both of his birth parents

You need to find the adoptive name.
Only the adoptee, SS or a registered adoption intermediary are able to gain access to the adoption file to find the name change. If you employ an intermediary, check their charges before committing yourself. Worth adding your details to the Adoption Contact Register..
Good luck.

Hi
Thank you, I will pass this onto Chris.
Unfortunately his birth Mother seems to either not remember, or chooses not to divulge, any additional information. Chris told me today that he asked his Social Worker if there was any information in his file about his Dad but he was told that there wasn’t.
It’s very sad.
Thank you again, you’ve been very kind.
Best Wishes
Sarah

Unfortunately , it's highly unlikely that either George Philips himself, or anyone who knows him, will happen to see your post here.

If your son-in-law knows which authority was in charge of his care and fostering, perhaps he could contact them in case they still have records which might give more info.

There were three babies named George Philips/Phillips registered in London 1955-65.
Two of them have middle names.
Does your son-in-law's father have a middle name?

Looking at the Electoral Rolls here on GR, and specifying birth 1960+/-5, there are 15 results for men named George Philips, with and without middle names, and literally hundreds with the name spelled Phillips. Many are duplicates, and many are not current addresses, but that still leaves huge numbers of possibilities - unless his name definitely has only one L.
But without knowing where he lives now, or the names of other people who might be listed with him, it's unlikely that you will be able to decide which is the right record (assuming he's still living).

Also, he may have opted out of the public ER, in which case there will be no up-to-date online record for him.

There is no record of a George Philips (with one l), born 1960+/-5, currently living in Sussex, according to the ERs on GR.

There are ER records (not all current addresses) for 6 men named George (as first name, not middle name) Phillips (two l's), living in Sussex.
One record each for a George with no middle name; George D, George F, George N, George T, and four records for a George R or RJ, who I think is the same person each time.

I’m looking for a man called George Philips who was possibly born in London in about 1960 but was in the Sussex area in the early 1980s.
My son in law was born in 1983 and has never known anything else about his Dad, beyond the name. He was taken into Care when he was 10 and thankfully he had great foster parents. However, over the years he has asked his estranged birth Mum for info but, presumably for her own reasons, she won’t tell him anything else.
I realise this is a long shot.....
Thanks