Instead of a codified constitution, the British government relies on a network of laws and conventions to place checks and balances on executive power. Conventions form the backbone of parliamentary proceedings. They are, after all, the source of Boris Johnson’s prime ministerial authority. Convention dictates that the prime minister is the person that commands the authority of the Commons, either as the majority party leader or the head of a coalition. 
There are two implications for this. Firstly, if Boris Johnson wants to do away with parliamentary convention, then he will have to find an alternate source of power as prime minister. He cannot accept parliamentary conventions when they suit him and discard them when they do not.
Secondly, because parliamentary conventions (along with laws) make up Britain’s uncodified constitution, breaking parliamentary convention is tantamount to breaching the British constitution. Therefore, in proroguing parliament for an unprecedented period of time, Johnson has violated the constitution, making the act unlawful.