In what way is he right?
Take his volcano claim -- if it's a supervolcanic eruption, he might well be right, but then in such circumstances we'd have more to worry about than the CO2 emissions. At current levels, though, volcanic CO2 emissions
in total are reckoned to be less than human-sourced CO2 emissions (per year in both cases). There is some confusion about this because it seems that different people come up with different figures; I reckon the confusion is because on average volcanos emit less, but a pretty big eruption will emit a lot. However large eruptions are also rare, so the accounting comes down against volcanoes.
See, for example,
http://hvo.wr.usgs.gov/volcanowatch/archive/2007/07_02_15.html
But regardless, it doesn't matter which is dominant, and once again a climate skeptic is failing to take into account the signal-to-background analysis that is important here. Natural emissions in total can, and probably do, dominate. But there was some level of balance, as CO2 sinks such as plants and forest life, and long-term CO2 sinks like coal and carbon, counteracted the emissions. Human activity is disrupting that balance, by destroying the sinks (large-scale deforestation) while simultaneously increasing the sources (burning of fossil fuels/ large-scale animal farming/ paddy fields, all of which also emit CO2). The effect has been to tip a balance from something like 100 in, 98 out to 103 in, 96 out. Do not take these numbers too seriously -- I use different ones every time -- but the point is that it doesn't matter how small our contribution is relatively, it is signal over the background that in both directions is increasing the level of CO2 in the atmosphere, and inducing extra warming and extra climate effects on what there would naturally be. We can -- and should -- try to combat this.
And other gases, for that matter, most notably CFCs whose greenhouse effect is orders of magnitude greater than CO2's is, while also being entirely artificial.