It is true that the UK rebate is not set in the EU treaties. It is a settlement, negotiated by Margaret Thatcher in 1984. The rebate reduced the UK contribution to the EU budget from more than 20% of the total in early 1980s to about 12% of the total now.
The level of the UK rebate is decided every seven years, as part of the EU's long-term budget, the Multiannual Financial Framework (MFF), which is negotiated by the EU leaders.
The long-term budget determines EU spending levels and priorities and it has to be approved unanimously by all 28 EU leaders.
Last time, as part of the 2014-2020 budget negotiations, the UK rebate was preserved. It will remain in place in the current form until 2020.
When the EU starts to negotiate the next MFF, the rebate will certainly be on the table again. Many other EU countries would like to see it scrapped or reduced.
But with the EU leaders required to approve the MFF unanimously, the UK could use its veto to stop the erosion of the rebate.
Reality Check verdict: He's right that the rebate is not a permanent feature of EU membership, but the UK has a veto over the process that would scrap or reduce it.