I've found several people in the CA Birth Index who were born in San Diego around the same time with the last name Simms and mother's maiden name Erving/Ervin -- might be his siblings.
Leon Simms, 16 Feb 1939, MMN Ervin
Gene Arthur Simms, 19 Nov 1943, MMN Erving (died in 1984)
Don Simms, 25 Jul 1945, MMN Ervin
Lerry Alvin Simms, 1 Nov 1948, MMN Ervin
I found Leon in the 1940 Census, living in San Diego with his mother Emma Simms and grandfather 'Gardia Ewin'. According to that census, Emma was born in Texas around 1916 and her father was born in Texas around 1896. It indicates that they were living in Live Oak, Hunt, Texas in 1935.
OK, I've found Gardie's death record. Full name Gardie Vernice Ervin, born 18 Dec 1894 in Texas, died 31 Mar 1959 in San Diego. There's a WWI draft card for him and some veteran's stuff (he's buried at Ft. Rosecrans National Cemetery), but I haven't been able to find him in any earlier census records.
I haven't foumd a death record for Emma. She might have remarried. I did find several articles in the SD Union (through GenealogyBank.com) about an Emma Simms, who had a run-in with the law in 1962.
Hope that helps. Please keep in mind that this is all guesswork. The people I've found might not be the Simms/Irvings you're looking for. Hopefully, the birth certificate you've sent for will be more definitive.
Best of luck to you,