I presume that you've already checked the horn to determine that IT isn't a problem.
Then just two recommendations:
1. Long tones -- working on descending to lower ones. But work on them slowly to the point that a given note can be produced immediately with correct embouchure.
2. Mouthpiece. What's he using? Very frequently beginning tuba students are given mouthpieces that are simply too large for them. (In fact, often tuba players use mouthpieces that are too large for them, believing that "bigger is better" or "bigger is easier".) If I had a beginning student in a similar situation, I'd start him on a Bach 25 mouthpiece. If he's using a Bach 18 or something larger (like a Conn 120S), go to the Bach 25. You can get a Kelly 25 for around $30. If he's using the dreaded 24AW, toss it so no one else can find it and use it. Once he feels comfortable with good long tones in the middle register and down around the F at the bottom of the staff, then he should easily be able to go lower with easy practice.
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)