What about a star chart or something similar to praise him when he does go to the loo? And I think the few days of toilet-training would be good too if you can do it. Though from what you say he is quite capable of actually going to the loo. But maybe the constant praise for catching him doing the right thing for a few days will make the difference. Can you get someone over to help you with Tom and you concerntrate on Jack for a few days, like your mum or a sister...? Maybe your DP could have a week off or something??