Siri won't be able to simply use dictation without the prior command of telling it to dictate. Siri will only be available for the 4s so he would have to upgrade.
Dragon Dictation is free and it's the best in the app store, but it's only ok if you ask me. I have a deep voice and I have yet to find a voice controlled software that actually works well in a public setting. (Crosses fingers for Siri to work!)
Why not try a sound amplifier and a pair of headphones for yourself? Something like uListen ($4.99) or Volume Up ($0.99). That way you won't have the confusion of the software typing the wrong thing.
If you decide to use the amplifier, make sure you use earbuds and not earphones or over-ear. That way the headphones actually create a sound tight seal in your ear canal and you won't lose any sound. Skull Candy makes very good and inexpensive ones. $20 for earbuds, and $30 for earbuds with inline mic and volume up/down and call buttons.
Edit:
I read my post to the Dragon Dictation app and this is what it came up with:
Seria Molby I would simply use dictation without the prior coming on the telling it to dictate serial will only been available for the for us so he would have to upgrade Dragon dictation is free and it's the best out in the App Store. It's only okay if you ask me all the deep voice and I have yet on the voice control software that actually works well in a public setting, why not try song amplifier on a pair of headphones for yourself something like you listen for for hunting on our volume up for $.99. That way you won't have the confusion of the software type in the wrong thing if you decide to use amplifier make sure you use in ear headphones on that ear buds for over year