Formula to whole milk

My baby turns 1 on the 13th may. My current tin of formula finishes by tomorrow. Is it a big deal if I just start him on whole milk ??? Should I buy 1 more tin of formula? He is a great eater and sleeper. No issues at all.

Update:

Thanks everyone. Last night I finished my last tin of formula and just started the whole.milk today and it was like nothing! Thank you all!