If he doesn’t have a soundcard then a synth is useless unless he just wants to jam on it stand alone without being able to record it.
That being said,
The Korg Volca series are great. The Bass, Sample and FM being the best of the bunch. If he likes his Maschine, he’ll like the Sample – it’s a sampler as the name suggests. Might struggle at £70 but ebay might be lucky
Teenage Engineering Pocket Operators are cool slightly DIY slightly quirky ‘toys’ but still are cool:
https://www.teenageengineering.com/products/po
£50 new, im sure you can find a bargain.
Virtual synths are another ballgame that would require knowing a bit more about the music he makes as some things would be downright useless and have no utility depending on what he’s into.