is he long allover, or mainly long legs?

if he is short torso, but long legs, i am guessing separates might fit better (different sizes)

heaps of boy coloured leggings around. Plus there are heaps of boyish Huggaluggs and babylegs to pick from (legwarmers).

To get bottoms long enough for extra long legs, i would try buying bottoms (e.g jeans) that are a size too big, but have an adjustable waist (which you might have to put on the smallest setting). big w has quite a few like this.

and for summer, buy the "long short" instead of short shorts. again in one size too big, but with adjustable waist.

If the whole onesie thing is too complicated, in the very early days, an alternative might be to swaddle instead.

the benefit of swaddling too, nothing to be "untucked"

in general, i would upsize EVERYTHING. So tops hang longer, bottoms end longer.

but i am saying this, without really understanding if your bub is LONG TORSO and LONG LEGS, or just LONG LEGS.