If you were single then you may be able to make a claim. However, if you are married, or were living with someone during the time your son was living with you in 2016, then you are considered common-law as of the date your son moved in, regardless of how long you and your partner were living together previously.
Also, if your son's mother was single, at least for the period of time your son was living with her, then she may also make a claim. However, only ONE of you are able to make this claim. To avoid conflict, speak with your son's mother. If both of you try to make a claim, and both of you qualify otherwise, CRA will deny the claim to both of you until you can come to an agreement.
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/305/lgbl-eng.html
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/305/sttns-eng.html
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/305/q7-eng.html
