He will be fine eating from tongs life-long. Not sure how big he is, but 15-20 regular mealworms is fine for a decent size leo. I hope you are dusting them? Dark stools can mean nothing at all, if occasional. IMO given the situation he's in, it is most likely due to his digestive system getting back in full gear. I am seeing the same thing from my leo who just finished a round of antibiotics, and I'm thinking it may be due to a slight imbalance in her gut. If it doesn't right itself soon, I'll be trying some probiotics, something I'd recommend for yours as well. You can find them online, someone, maybe ESU?, makes a probiotics specifically for herps, tho I have never used it yet.
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1.0 Hypo stripe, Het RW from JL (BJ)
1.0 HypoTang from Crested (Apricot)
1.0 Tremper Albino from Petsmart (Cloud)
0.1 Hi-Yellow Leopard gecko from Petco (Beatrice)
0.1 Stripe female (rehabilitated rescue)(Pepper)
0.1 Jungle het RW hatched by me! (Jungle Bunny)
0.1 SHCT Leopard Gecko from Petco (Brite)
0.1 Tangy Mutt Leopard Gecko from Petco (Rainbow)
0.1 RW Stripe hatched by me! (unnamed yet)
0.1 Leucistic rescue (Lucy?)
0.1 Tremper stripe from JMG (Delta)
0.1 SHCT hatched by me! (Blinky)
0.0.1 SHCTB hatched by me! (Sunny II)
0.1 Ball Pythons
1.0 Brazilian Rainbow Boa
Hatched: ~50 leos