I'd get him seen. I'm a big believer in letting them take there time and after four I can say for sure that they all do things at their on pace. My dd was born end December do almost the same age. She has rolled four times in her life. She doesn't really crawl but can get on all fours for ages. She can sit. She Islas also started to pull herself up to stand. She can't not do it she is just super lazy as she has three older kids that do everything for her.
Sounds like you little one could use done exercises. That same nurse should be able to refer you to the hospital physio and that way it will be free