Birth records at PA State start at 1906. Before 1906 some counties have some records. Otherwise you would have to find a baptism in church records.
The tombstone for Henry Clay states “His wife Lou”, therefore, the following could be his marriage as Lou could be a nickname for Louisa.
Pennsylvania, Church and Town Records, 1708-1985
Name: Henry Clay Davis
Event Type: Marriage
Marriage Date: 9 Feb 1871
Marriage Place: Philadelphia, Philadelphia, Pennsylvania
Spouse Name: Mary Louisa Dixey
Organization Name: Church of the Advent
Original Record of Marriage shows: February 9, 1871; On Thursday Morning I solemnized the Marriage between Henry Clay Davis and Mary Louisa Dixey. Signed J. W. Claxton.
There is one person named Henry Clay Davis shown in the 1850 census in PA:
William W Davis 42 born Wales; miner
Henrietta Davis 30 born PA
David R Davis 18 born Wales
Gomar S Davis 7 born PA
Henry Clay Davis 1 born PA