Child Passport Change of Name

[ALANY76]

Hello all.
my son born in uk 2005 , at that time he was registered under different man name on birth certificate ,
at the time of born mother was refuse case father was british , so he apply for my son and he got uk passport ,
but in 2008 i made dna test come back he is my son , i send test result to passport office with new birth certificate , the issue new passport for him under my name as father .

now in 2017 i made new application form for him to change his surname to my new surname , ( i just change my surname last two months ) now passport office send letter to said you need to apply for new naturalization for your son because the passport was issue under other man you you are father now ,
please advice what can i do now .?
my son born 2005 and i have up to date new birth certificate im father , but at the time he born i was not marry to my son mother ,