Immigrated as in receiving a green card as lawful permanent resident?

Which country is he a resident of?  US or Mexico?  There are people who receive a green card but live overseas (albeit "temporarily").

What kind of pension is it?  Private pension, pension from government services, or social security?

He is a Mexican that  Has Resident Alien Card and he is receiving retirement pension 

Would like to help but you will need to answer those questions precisely as they were asked.

Mexican social security paid to your client, who is a tax resident of the US, will only be taxable to Mexico and exempt from US tax, pursuant to Article 19(1)(b) of the US-Mexico DTA and on the basis Article 19(1)(b) is excepted from the Saving Clause in accordance with Article 1(5) of the 2003 Protocol.

Thank you very much for your response, and do I still have to report this money like we do with the 1099 SSA?

It doesn't need to be reported.  Treaty disclosure generally required under §6114(a) also is specifically waived by §301.6114-1(c)(1).

Thank you so much for your help.