I failed all juscical aptitude tests at primary school, and I didnlt start learning the Cello until I was 14.
I now have a music deghree and I teach the cello.
Don;t worry about DS1 not being "rhythmical" or having no technical skill, as that's why he will be having lessons!
I have taght kid (an adults) with absolute;y NO musucal ability at all, and, like everything else, if he works hard at it, he'llbe fine.
I think it is best if you get your DS inbvolved in choosing his instrument, as then he iwill be mpore likely to stick at it for longer. IME, the kids whoi quit are the ones "forced" into it. He is only in y4, so presmably 8/9, which is a good age to strat an instrument, but if you were to wait another year, it wouldn't be too late. If he shown no interest in learning at this stage, then do as other sufggest and ge him interestyed ina choir oir something (especiually if his chums do it too) He may well want to learn an instrument as a resuklt of this, esp if he sees other kids playing in orchestras etc.
To help him choose, listen to recordings of each oif the instuments, and see which one he likes the sound of best. Also, it might be helpful to see if there is any videiopos on You Tube or somthing of these intruments being played so he has a feel for it.
The most importnat thing IMHO is that he id the one who wants to do it.