He learns it by breeding, but it looks like no compatible Pokemon learn Ice Punch in gen 5. So you will need to teach it to an Electivire in Platinum/HG/SS from a move tutor.
Or, teach it to one of these Pokemon and breed:
- Abra
- Machop
- Drowzee
- Hitmonchan
- Mr. Mime
- Makuhita
- Sableye
- Meditite
- Volbeat
- Spinda
- Lopunny
- Lucario
- Croagunk