Just as Zonne and DaisyAnne say. Johnson is mistaken if he is claiming that leaving the EU is what allows this change:
In 2007, the EU commissioner Gunter Verheugen confirmed that the Crown stamp could be affixed to a glass provided it was done in such a way that it was not confused with the CE mark.
However, the reason the Crown symbol could not be placed on pint glasses which carried the CE mark was due to the UK’s own legislation: Regulation 21(2) of The Capacity Serving Measures (Intoxicating Liquor) Regulations 1988 which states:
No capacity serving measure shall be stamped if it bears any other mark which, in the opinion of the inspector, might reasonably be mistaken for the stamp or an expression of approval or guarantee of accuracy.
The British government could, at any time, have repealed or amended Regulation 21 and reintroduced the Crown mark without having to leave the EU to do it.
In other words, it’s another of those misleading taking back control, taking back our sovereignty claims; blaming the EU for something the UK always had control over.