It really depends on the brand.
Clothes 3-6 months from Primark are getting tight.
Usually he is wearing his 2-4 months clothes but in come cases 4-6 months fit well also.
He is now approx. 24 inch and weighs 13,9 lbs
to compare, my daughter who is 2 years and 3 months is 22 lbs and around 33 inch
For size diapers he has size 3 (midi, 8,5 - 20 lbs) and my daughter is wearing size 4 (15 - 30 lbs). But I am sure they will have the same size soon. My daughter only wears them during the night now 'just incase' because she is potty trained during the day and her afternoon naps.