I'd say get something he'll like now… maybe a light up toy or teethers. Then think ahead for the rest of the year. A walker, lots of books, little infant suitable toy cars, etc. I don't know what your other kids have though. If you don't have one, maybe get some sort of jumparoo type thing. We had a doorway jumper but it spins around too much. The stable ones might be better.
At 4 months my LO loved his Glo-worm toy. I think it's Fisher Price. It plays a tune and lights up.