He really should go - it's hampering his child's social life if he doesn't! There's a disconnect, sure - women probably won't start conversations initially, possibly, my DW guessed, for fear of being seen as the one chatting up the dad, but it's amazing how toddlers fighting over toys gets people apologizing, then chatting...
I did it for nearly 6 years (DS [#2] is still preschool but now just a bit too old for toddler groups) and found mums more chatty at some groups than others; but at those where they weren't, new mums (whom I already knew) who came along noticed the mood was standoffish and not particularly friendly anyway.
The people running the groups are usually chatty and friendly (and they're the ones in control of the biscuits and coffee - so good to be alongside them anyway). Helping out is also a good ploy - offering to lift and put away climbing frames and other big stuff (whilst IGWS you lot are eminently able/capable) is usually quite welcome, and the day the coffee lady was ill and I had to step in meant EVERYONE had to chat to me. Ooh the power.
Clearly I don't know if anyone thought I was a perv, but I doubt it. IMHO at some groups it was women checking out how each other looked - I, and the other blokes, were too busy refereeing Lego wars or changing nappies.
One thing tho' - if your DD is still tiny, any conversations with more than two new mums is likely to turn to episitomies and VBACs etc - if he's in the midst of this, he needs to be alive to the signs that they're heading in that direction and go and change a nappy or something before it all gets a bit uncomfortable all round.
A bit long - but a view from the other side - HTH