I think that father and son are not really interested in seeing each other. The father does not take active steps to contact his son and is not contesting sole custody by my fiancee. I realise that 'sole custody' is not the same as 'sole responsibility' and the visa authorities are really concerned with 'sole responsibility'.
But the boy's father has had no input into his child's life since September, 2009; he has made no payments to my fiancee to financially support him and is not involved whatsoever in decisions over his care and schooling. I do not even believe he has even made any offer to make payments or take an active part in decisions regarding his son. My fiancee has never sought any maintenance payments and she is not seeking any share of the property they once lived in or any other payment from him upon annulment.
The only moneys my fiancee receives are her own earnings and what I send her each month. The money I send certainly accounts for more than half her total income and I have kept a full record of these payments. I think this would be enough to show that I have taken on responsibility for my fiancee's son.
I guess the visa authorities may well want to contact the boy's father, but even while my fiancee is in the UK, he would be cared for by my fiancee's family, and the father would still probably not want any input into his life.
I do realise, however, that if the boy's father did know of my existence, he may change his mind and want his son back.
If there were visa problems, my fiancee could always go to the Philippines to collect her child.