All I can suggest is plenty of practice I'm afraid.
About 10 of my pupils have the same problem (and my own DD). Handwriting can never keep up with the speed of thought. They just have to learn to organise their thoughts and practice their memory skills.
I allow a piece of scrap paper so they can jot down quickly a few key words which will help them remember what they wanted to put. This is useful for helping structure the writing too. Pupil J produces the most complex jottings with arrows in all directions, I said he should frame and sell them
Unfortunately, when a child finds they are faster at typing, they will always use that for preference. This means their typing continually improves while their handwritten work stagnates even further as they are never using it when typing is an option. The only solution in that situation is to reduce the times typing is an option. Keep a balance so that he does not lose his typing skills but practices handwriting his work more often to improve his thought organisation and speed there.
I have to say also, that despite their preferences, they all need to learn how to write fast by hand or they just will not get enough completed during their exams.
One of my Uncles marks Exam papers and says the biggest thing that lets pupils down is their writing, a) how easy it is to read - he cannot give points when he cannot read it and b) How much they can actually write in the available time. DU says it is quite shocking how much less current pupils seem to manage to record in comparison to those 20 years ago.
He and his colleagues get very frustrated by this because DCs are not getting the marks they deserve purely because of lack of practice in handwriting.
He believes, quite logically, that it is due to the prevalence of computer use. People forget how to handwrite clearly and fast.
This is not helped by many secondary schools encouraging word-processed assignments (they are not allowed to insist - that is illegal, but they certainly "strongly suggest").
As a side issue, he bemoans the deterioration in spelling from DCs who seem to rely on their computers' spellcheck most of the time (particularly when they absorb the American spellings which often come up).