Generally I'd say, he's 5 months old, give him some more time, but if you have a pet sitter coming to let him out twice a day, I'm puzzled. I'd be wondering if she is really doing her job and exactly how long she is giving him to try to go potty.
Just my suspicious mind at work...
I have to agree. with a sitter coming over to let him out, he should be able to hold it till you get home. Maybe the break in the day is upsetting and confusing to him. At 5 months, he should be able to go a full day in the crate.
I say he's acting out for some reason. Maybe you should get a baby cam and see what is going on during the sitter visits.
but at 5 months, he is still young and learning. he will mature in time.
Does your crate have one of those dividers? When Grace was little, we used the divider. She would potty in the crate, but only without the divider. given the extra room, they will take it and still have a dry spot to sleep. As they get bigger and need the room and you remove the divider, they stop making messes in their crates because there's really no room. If Lady had a tummy issue and needed to go she would actually push her behind against the crate and shoot it out onto the tile floor so it wouldn't be in the crate with her.