Not in your shoes, but in hers. I had a termination at 16. It was the right decision for me.
Ultimately its not about how you feel; they've made a decision so the best thing you can do is support them.
They both need support, but she will need more as she'll have to deal with the physical aspects; hopefully her mum / family will help, but she'll likely want support from your DS too as it sounds like they are working through this as a couple.
Your DS may need support in dealing with emotions; his own and other people's.
Help with the practical arrangements if you can; driving / taxi fares to appointments etc. Be prepared to walk them into the clinic. Walking past a fucking protestor alone was shit.
Help your DS to be supportive emotionally but also with some comforts afterwards when she may be in pain and need to rest - chocolate, cosy darkcoloured PJs, a voucher for UberEats for a takeaway, Netflix or something for films.
Time and company is helpful, & help them consider who they want to know; my mum inviting my Catholic friend over a few days afterwards to cheer me up was not helpful as I knew they had strong views against abortion.
Make sure they are aware of & have access to reliable contraception afterwards - you can be fertile very quickly, so as much as it feels like something to think about later, it's good to sort out ASAP.