I have six, although one has left home so technically only looking after five.

First thing: extracurricular activities. He is worried that we would be broke from having them all play something (and with what could prob be four boys theyd most likely all play football) and that most of our spare time outside of work hours would be spent ferrying them here there and everywhere. Thoughts?
Some associations give you a discount for subsequent kids. But, yeah, it can get expensive. Uniforms, fees, etc. You can get to know other parents though who can help out with spares, and I share netball on Saturday mornings with other parents, so I don't have to ferry dd#4 every week. Dd3's music lessons are after school and she can get herself there and home. To be honest, I don't necessarily think that parents *should* ferry their kids everywhere unless there's no alternative or they are too young. When they are older, they can ride bikes, catch the bus, etc. We also have limits on what they can be involved with - one sport, one other extra curricular, like music lessons, Scouts, etc.

His second thing: financial costs for feeding six people, and possibly four boys. With what I cook now, I definately have enough left over for an extra meal that usually gets eaten as lunch the following day or frozen for later. Thoughts?
Our food bill is about $250-$300 a week. It could be less if we tried harder. I do menu planning, and I try to do some cooking in advance (and I'm also including in that helpings from each meal for my dad who is an invalid, and I freeze a ton of meals for him each month). We are thinking of going to a couple of vegetarian meals a week for our health, but we're also happy about the impact it will have on our budget! I think you are right with the shop smarter idea, and the making your own snacks. But I can't recommend menu planning enough - it really does help you get the absolutely best value out of your food.

I just dont think one extra kid at this stage will impact all that greatly on our lifestyle. Three is already hectic, I am sure another child thrown in; while adding a bit extra busy-ness, will not make or break us. If its another boy even better, he will have untold amounts of perfectly usable clothing from his older brothers. We would just have to be savvy in optimising group discounts and the like...........
From a workload point of view, I didn't find #4 that much of a big deal either. Where I *did* notice the difference was in things like, family deals for entrance to things is always two adults, two kids, sometimes two adults three kids, but never EVER two adults four kids! Cars to fit four kids are more expensive. It's very hard to find hotel/motel accommodation for a family with four kids. But, we still have holidays, we still go places. With the hand me downs, clothes might go through two kids, even three, but by the time they get to #4 they can be a bit run down!

Where I really notice it is with four at school at the same time. The school fees, uniforms, books... it's a terrible time of year for us financially, especially coming just after Christmas. And pocket money. The kids all grow up knowing that when they hit sixteen we expect them to get a job to start looking after their leisure stuff by themselves, to ease the burden a bit.

My brutally honest opinion is that if you have to work really hard at convincing your dh to have another one, he's just going to keep on coming up with endless other reasons. It may be that he's just making excuses and he feels like he's done. And that's a harder one to answer than the practical questions you've asked.