John has put you along the right path.
On the 1891 census for Gravesend is this family:
George William Johnson 36
Emma Johnson 37
Ada Johnson 12
Alfred George Johnson 9
That puts Alfred George as born around 1881/82, which fits with the reference John quoted (Johnson, Alfred George - Gravesend - 2a/465) That reference can be used to get a copy of his birth certificate from the General Register Office (www.gro.gov.uk
) The certificate will confirm his date of birth and also the spelling of his mother's name (or at least the spelling used at that time.
Intriguingly, there is a marriage for an Alfred George Johnson in March qtr 1903, to either Annie Atkinson or Alice Minnie Mason, also in Gravesend. The place a date (he would be about 21 yrs) make it a possibility that this is your grandfather, though by no means certain. It might be worth getting that marriage certificate just to check. Also look for possible Johnson deaths in the following years to find out what happened to that wife. Could it be (heaven forbid) that he ran away with Letitia?
This info mostly from www.freebmd.org.uk
(I have Johnsons in my family from Kent, but not as far as I know related to yours :-( )