I'd go with standard velocity service-type ammo with SWC bullets. More punch than a WC, but the way everything seems to be under-loaded today, almost no difference in recoil.
Kaaskop49
Shield #5103
Kaaskop49
Shield #5103
Did you consider the light bullet Hornady Critical Defense loads in .38 Spl?
Hornady Manufacturing Company :: Ammunition :: Handgun :: Choose by Caliber :: 38 Special :: 38 Special 90 gr FTX® Critical Defense® LITE
Hornady Manufacturing Company :: Ammunition :: Handgun :: Choose by Caliber :: 38 Special :: 38 Special 110 gr FTX® Critical Defense®
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I agree with the several posters above recommending a heavier gun. A steel J frame (36, 640, etc.) might be the ticket as might a Model 10 snub. A reduced recoil .38 load in a steel frame would be far preferable to any rimfire. The Airweight is not kind to sensitive hands.
148gr WC .38 Spl should be bearable if he has the strength to use the 642 at all. Beats .22 any way you figure it.
Putting a larger j frame grip on the 642 could also spread the recoil and reduce the sensation.
. Astonishes me, considering I am probably smaller than all of them and don't have an issue with any caliber.