Poo regression or just lazy?

[Halfpastfreckle]

My son is coming up to 3 in sept. I wanted to potty train him this summer after our holiday abroad but 10 days before going he announced to me he wanted big boy pants so we went with it and did the potty training. It went pretty well and only 2 wee wee accidents on holiday both when distracted. On return he is now out of nappies at night and not once wet the bed. Poos were never great he would crouch down and then we’d whisk him onto potty or toilet and then he would go. So obviously I think he is totally ready to be trained doing pretty well all in all and not afraid of doing poos on the toilet. We had got to the point where a couple of times he actually said I need a poo poo and did and then did it so we really thought we had it cracked. However this week (so about a month/5 weeks in) he has seemingly decided that poos are for pants. He has pooed in his pants every day this week and has done a few on the toilet only by being vigilant and getting him on on time. No wee wee accidents at all btw. He has just now leaned forward at the dinner table whilst laughing and joking and eating his dinner and done the biggest poo into his pants. I’ve gone back to the Gina ford book but no help! What do I do? I feel like he’s just being lazy or defiant or something? We’ve both been quite stern with him tonight and told him that was so naughty and he knows better - was this the wrong thing to do??