Should we stop or carry on?

[twolittleboysonetiredmum]

Potty training ds2. He asked to start using potty and wear pants so completely prompted by him. Had 3 successful days - 1 or 2 accidents - and even 2 poos. Day 4 he starts poo withholding and refuses to go on the potty by the end of the day as he's so scared of pooing. Day 5 (today) was a nursery day. He's managed one wee oh the potty and had accidents (including small poos) most of the day. They've sent him home in a nappy (I'm not happy about that but can see why) and he's refused to use the potty since. He's exhausted as wouldn't nap so could be he's just in a grump.
Should we just stick him back in nappies or persevere? I'm happy to do either but it seems a shame to stop too. I suspect nursery weren't encouraging him enough to go regularly so could have a more detailed chat with them tomorrow. But I don't want to force him to train either and cause him longer term issues. Any advice?