I would take the initiative here and not wait for him to "tell you when he's ready" and take the Pull-ups away, use underwear and cover them with waterproof pants or buy the waterproof training pants in one. The initial cost may seem like a lot, but you're spending that on Pull-ups anyway over time, which are expensive.
Wearing these he will feel when he starts to pee or actually does and can head to the bathroom on his own. Have him use the toilet before he sits down to the computer and every 20 minutes or so while playing. And, have him clean up after pooping in his pants.
At 3.5 I'm wondering if you've asked why he won't sit on the toilet? My 26 month old is slightly embarrassed to poop in his potty chair, but he DOES sit and do it then says "Oh oh" and I make a big fuss about what a great job he did : ) Have you tried a potty seat on the toilet or just the regular seat? It's hard to know why he won't, but at this point it's a power play and I would try to find out WHY and deal with it..
You or dad can take him to the bathroom when you go, and have him sit at the same time if he has a potty. Let him see that pooping on the toilet is natural. Also, if you have friends or relatives with children around his age who are trained let him spend enough time around them to see that they use the toilet, peer pressure can be a good thing.
Another mama posted this yesterday and I found it very helpful, so I hope you will benefit from it as well, 10 Techniques to Toilet Training ~
http://parentingtouchstones.com/01/10-techniques-to-toile...