“So, should his Mother claim him the same as she does every year or should he file as head of household?”--->As long as his mother claims him as her dependent on her return, he(the son) can never file his return as HOH;to file his return as a HOH, he must be single and have a qualifying child/qualifying relative who lives with him for over half the year and doesn't support himself or herself.Also, to claim him(18 YO son) as her dependent, for her 2011 return, the son , as her qualifying child, had to live for more than half the year, the son must have the same residence as her mother does. As you said, “During 2011, my Son turned 18 and moved out on his own.” As long as she lived with him for at least half the year and meets other rerquiremnts,i.e, support, age or etc, she can still claim him as her dependent. The son can’t file joint return with his 17 YO GF; as GF is NOT his legal spouse and they are NOT married.Also, his GF’s mother can claim EIC on her daughter, your son’s GF as herdaughter was under age 19 at the end of 2011 and must have lived with her for more than half of 2011. She can also claimgrandc hild for earned income credit if she meets the income criteria.
“If he can file on his own, can he claim the girlfriend and daughter? Does he qualify for the special credits?”---->UNLESS he is claimed as his mother’s dependent, he can claim his return as HOH as long as he cared for his daughter, a dependent, for over half the year, 2011. If hid GF also claims the child, he can NOT file as HOH.He can never claim his GF as a dependent for HOH.However, he can claim his GF as his dependent on his return as HOH as long as(as said previously) he meets the conditions ,i.e, support, her gross income or etc.
“Son 18 :• Lived with Mother prior to 18th birthday (July 25 2011)• Lives on his own (since July 25) • Has full time job (since July 25)...student prior to that”---->His mother can claim him as her dependent on her return as she has lived with her son for more than half the year in 2011 and as long as she meets other requirements, i.e, support or etc.
“• Has 2 dependants- 17 year old girlfriend and his daughter (born Jan 2011)...both live with him (since July 25)”---> As said, as long as he is claimed as a dependent of his mother, he can’t file his return as HOH; however, as her dependent, he needs to file his return as single, NOT as HOH.
“Girlfriend 17: Lived with her Mother prior to July 25 2011• Lives with and depends on my Son since July 25 .Grandchild <1 : Lived between Grandmothers prior to July 25 2011• Lives with and depends on my Son since July 25”----> Her mother can claim EIC on her daughter/GD as long as she meets the condition as said above.