eeuroparts is showing the genuine 975546 M12x35 bolt in stock, possibly just meaning available from a distribution warehouse. ipd is showing it as avail to order. It’s also shown avail on foreign sites like scandix. When searching for Volvo p/ns you should omit the -3 check digit suffix. Fitment is listed in Volvo parts catalogues as 1988-on 240 w/Girling front calipers (which all were).

Genuine Volvo parts sites are showing this as a flange screw with a partial threaded shank, which is what I vaguely recall is original. As long as it’s grade 10.9 so it can be suitably torqued without stretching and long enough to fully thread then I wouldn't get too excited about the shank or the flange (I'd suggest a flat washer if there's no flange, but for calipers you don't want to encourage movement). A drop of thread locker is called for, but I never bother, one winter of salt corrosion will more than suffice.

The odd Volvo dealer parts & service may still have a couple in old stock as they technically weren’t supposed to be re-used, although I'm sure most shops did. I’d suggest approaching a friendly bone yard and see if they’d be willing to sell just a couple bolts. A good Volvo indie may even have a couple spares in a box. Pleading innocence because your son lost them may help.

Local fastener distributors will generally want to sell you a box full as it will likely be a special order. McMaster-Carr has the flanged screw under p/n 98093A756 in qty 10. A quick search under that p/n suggests it may have been used in some Ford and GMC, so you could always try those parts houses, but I wouldn't get my hopes up of anything in local stock. Fastenal has the bolt under various p/ns depending on finish and quantity, including individually, but doesn't appear to carry the flanged cap.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now