I found it quite easy. There are a LOT of similarities. The euphonium is really a baby tuba that just hasn't grown up. (Euphonium players tend to dislike this description and insist that the euphonium isn't a tuba -- but structurally it is. It's just the baritone voice rather than the bass voice.)
In fact, in some fundamental ways (certainly as a beginner) the euphonium is easier and more pleasant to play than the tuba. The fingerings won't all be identical (since the euphonium is pitched an octave higher than the BBb tuba, though both are pitched in Bb), but most of them will be, and he'll adapt to the minor differences almost immediately. He should not have any trouble with the smaller mouthpiece and will find it easier to get a decent sound out of the instrument. At least that was my experience, and I suspect it's the same for others who have done this as well.
To some degree this assumes that he ends up with a decent euphonium in good shape. But otherwise the transition should be uneventful. It might even be possible to play BOTH in his band if he's having trouble with Sousaphone: tuba in concert band, and euphonium in pep and marching band -- if the director will go for such a deal. Switching back and forth is a piece of cake. If the band isn't short of tuba/Sousaphone players, such a deal may be possible. But if the alternative is that the band will lose a tuba player anyway, the director also may compromise. Worth a try.
Last edited by ghmerrill; 10-01-2017 at 09:49 AM.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)