Jayden's Lump

A few months ago, I noticed a lump under Jayden's arm by his armpit and between his breast. It was scary so I took him to the doctor. The doctor told me it was a swollen lymph gland probably from an infection. Well this lump seems to still be there or has possibly gotten bigger. It is like a marble under his skin and it sticks out. It moves around. Mike took took him "along" to the doctor with the girls on Friday. The doctor didn't have his chart, but again, stated not to worry unless it stops moving around, feels warm/hot to the touch, or turns red. I was telling a couple I work with about it and he's worried. He has had cancer and it was a lump in that same spot and it wasn't red or hot. His was bigger. Anyhow, he begged me to take Jayden to get a second opinion and demand an MRI to see exactly what it is. Tonight I sit here and did some searching online and it seems I find the "not scary" stuff and the "scary" stuff where both sides tend to aim towards his lump. At this point, I'm not sure what to do. Our doctor has told us if it does become red, hot, or hard, then he would have to have it removed. It's just so creepy to see this bump coming out of him and even worse to feel. I'm just hoping and praying it's nothing and that it will go away. I'm probably going to call the doctor again and talk to him personally on Tuesday about why I shouldn't worry about it. How does he know for sure it's not cancer or a tumor? I don't know what to do!