My Son Has Stitches Inside His Lip Since October 29

My son has stitches inside his lip since October 29 (12 days ago).. ER told him to come back in 7 days in 7 days to have stitches removed which he did on November 4. Stitches on outside were removed, however, he was told stitches inside dissolvable type. Stitches inside have not dissolved today November 10(12 days since injury). Is this normal or should he go back to ER to have them removed?