My son 7mnths had 2nd dose of influenza vaccine 2 weeks bck. now m noticing brown medium sized spot on thigh.no swelling.should i be worried ?

Usually not: One usually does not need to worry if there is only a colored spot where a vaccine shot was given. The color may be from blood cells in the area of the shot. Because the needle pokes through things, there is a tiny amount of bleeding from the things that got poked through. Those blood cells leak out of tiny vessels, but the coloring should go away over a few weeks.