The more he writes, the faster he will get and frequently, the joining up just happens organically - switching to a comfortable pen that he can hold easily, combined with greater fluency of thought, speeds up that process.
I detested handwriting and cursive above all else in primary - I have hypermobility and it made my hand hurt, so the moment I could, I switched to block writing and was fortunately, allowed to do so. My printed writing got better, my hands grew stronger and the day I found that a cheapie Parker fountain pen set was comfortable, felt cool in my hand and allowed both the ink and my hand to flow as smoothly as my thoughts - because I was older and had developed the language and writing skills in my head over the years - I wrote quickly and fluidly in joined up letters. I was 15.
However, for some people, it just doesn't happen; but they also become much faster using block letters and in all honesty, it's most important to be able to read what has been written in adult life, not to have have a writing style that all too often falls into the stereotype of 'doctor's handwriting' or 'the death throes of a dying spider'.
He could see whether the 'easier' joins begin to flow - en, ugh, at, le - that could increase his speed without the almost absolute full stop that starting from scratch would cause. Or does he press very hard on the paper, use a scratchy pen? If so, a lighter touch with a softer feeling pen might help.
Or he could use a (chisel tip) fat whiteboard marker on A3 (or an A3 whiteboard, they aren't expensive on Amazon) for planning things like a list of homework to cross through once he's completed them - going big with paper and pen can be far easier to learn fluidity than a chewed Bic and he won't be 'doing MORE work', which is always a bone of contention for your average adolescent.