Is my dog as underweight as people think?

Pic: https://imgur.com/a/iMnpwMC

My dog weighs 50 lbs. He is a year and a half to two years old. His vet says he has a BCS of four and I think that’s correct based on my own understanding. She said his ideal weight is probably around 55 lbs.

I understand that he could stand to gain some weight and I’m genuinely trying. He eats four cups of Hills Science Diet Perfect Digestion per day and also gets treats. His fecals are clear and he has been dewormed twice just as a precaution. (I found him as a stray in September and he had a lot of loose stools before we switched to his current food.)

But people comment to me all the time saying he is emaciated and they have accused me of starving him. I trust my vet but feel insecure about how many random people seem to believe I’m starving my dog to death. If he really is as thin as people think, I want to get a second opinion or ask my vet for extra tests.

I’d appreciate if vets could verify that my own vet is correct that he is a little lean and could use some weight gain, but that he is not emaciated or in any kind of immediate danger.