A queen size flat sheet is great for the lego mess. Set him up on the sheet and when he is finished pick up the sheet by the corners and tip it all back into the bucket. At the age he is I would say go for it but only while you are able to play with him. Maybe hide it in your wardrobe or something to keep it safe.

We have stacks of Duplo and the girls all love it. We have even had it touching the ceiling. Do make sure you pack it all up as soon as he moves on to the next activity though, I have a lovely scar on my heel from discovering a piece with my full weight on the wrong angle. took 2 years to get that though.