PAC- from what you are saying it would sound like he cannot act towards his mother as a mother but she becomes a strange women to him like anyone else. she cannot sing in front of him….they cannot touch…is that true?
and
No way around it. (Unless he marries her.)
Rabbi Zef Leff disagrees with you. In the audio link below, he states that because they are still physically related, a convert is allowed to engage in yichud with his parents/grandparents/children and is allowed to touch them (including hugging and kissing).
[audio src="http://www.rabbileff.net/shiurim/answers/1250-1499/1372.mp3" /]
The Wolf