How do you know exactly when he was baptised? If you have seen the parish records didn't it note the father?
The only SUMPTER child I can see (on FreeReg) baptised at Foulsham around this time is Sarah, dau of Edward and Dorothy (13 Jan 1703/4). It looks like Edward SUMPTER (bachelor, yeoman of Foulsham) married Dorothy HALL (widow, St George Tombland) at Norwich Cathedral, 7 Feb 1702/3.