"Lo" is a direct object pronoun. They will be covered more in the rather difficult clitics section. But for now "lo" = "a él" here. "Yo lo recuerdo a él" = "Yo lo recuerdo" I'm not sure if the phrase without the "lo" is right or if it is it might not be common. The simple fact that "lo" is easier to say than " a él" means people will probably use it often. The "a él" just serves to clarify that it is not an "it" here. But if we already know we are talking about a "him" then it is just redundant.