I'm no expert but my husband and I do this, so will try to explain as best I can.
At first glance this looks simple. Your husband's gross income needs to be below 50k so the simplest was of doing it would be to pay the bonus into the pension every time it is paid. (Whether than be annual or monthly etc).
However you don't say if your husband receives any benefits in kind such as a company car, health insurance etc. If he does then he should receive a P11d. This figure is counted in the Earnings threshold and you would also have to make an adjustment for this to bring his earnings below the threshold. For example (ignoring the bonus for the moment), if your husband's gross income was 48,200 plus he had a bik/p11d value of, for example, 3,800, these two figures come to 52000, so you would have to pay an extra 2,000 a year into the pension fund. The easiest way to do this, rather than paying a lump sum, would be to pay an extra £167 a month into his pension fund. (If you start this part way through the year you need to make sure you pay the extra for the months that have already been).
The added bonus is that you should benefit from higher rate tax relief on the pension contributions too. There is an annual limit of maximum annual pension contributions of about 40k but that doesn't look like it should be an issue here.
I'm not sure that you will be able to do this retrospectively for the year ending April 2018, but should definitely be able to do it for year ending 2019.
Hope that helps.