Telecoms engineer Ollie Hall, 24, died in November 2017 after joining the Kurdistan People’s Protection Units (YPG) in the war-torn country to fight Isis.
The Gosport man had been inspired to fight against Isis after the Manchester bombing in May of the same year.
Now his mother Jane Lyndon has joined families of other fighters, international volunteers and academics in signing a letter to Sajid Javid.
It comes after the home secretary warned British citizens who are in northern Syria they must leave in 28 days or be jailed for 10 years if they return to Britain.
The co-signatories warn that he risks ‘criminalising’ people who supported the American-led efforts to overthrow the Isis caliphate.
At Mr Hall’s inquest the coroner, David Horsley, said: ‘His actions have made him a hero in the Kurdish community. He also should be an outstanding example of courage and self-sacrifice to whoever hears his story.’
Mr Javid is accused of failing to distinguish between the jihadist enclave of Idlib in the north-west and the Kurdish region in the north-east, The Observer reported.
The newspaper reported the letter said: ‘Using a law supposedly created to defend against Isis, you are criminalising as “terrorists” those who've given more than any other British citizens in this struggle.
‘Just two months ago, the whole world celebrated Isis's defeat as a military force.
‘That victory was led by the women of YPJ, along with the other Kurds, Arabs and Christians of north-east Syria - the democratic, women-led, autonomous region more commonly known as Rojava.’