Sounds like a combination of constipation and possibly some anal irritation resulting from it. Which becomes a really vicious circle after a bit as he holds on trying not to poo because it hurts, but that makes it harder to go and more likely to cause small tears around the opening. Any nappy rash certainly won't be helping matters.

TBH, I'd leave off with the creams for a couple of days and bathe with cotton wool soaked in cooled chamomile tea (sounds weird as heck, but it works!) and allow plenty of nappy off time. Couple that with some stewed pears or watered down prune juice to try and soften his poo.