The only mention of a special quiver for javelins is in the magic items list on page 189 of the DMG; the Quiver of Ehlonna.
This quiver has several extradimensional compartments to hold various ranged ammunition or similar sized items. Including up to 18 javelins or similarly sized medium items.
The Quiver of Ehlonna is only an uncommon wonderous item, so this solution isn't out of the realm of possibility to get pretty early in a campaign. Assuming you want to give out magic items of course.
The quiver in the Player's Handbook (Pg. 153) only mentions that it can hold 20 arrows. So, if you elect to give him a quiver that isn't magical, you would have to make the call as a DM as to the price and capacity of a quiver. Or just hand-wave the quiver away and say a person can only carry some number of javelins on their person that seems reasonable.