Oh wow - under 6 months! How sweet!
My son loved to be upright at that age. Are 'jolly jumpers' still 'legal?' He used to be able to jump in time to us singing.
Ours was sort of like theses -
http://www.fisher-price.com/fp.aspx...VADID=19153472012&OVKWID=59187093512but they hung from the molding over a doorway.
BTW - if you start making notes of the songs you sing now, you can get a good laught seeing how many years later your kid will still be able to sing it. File this under 'Stupid Gifted Kid tricks' I guess. Basically it's time for a news blackout, as they often store everything in memory, and use it against you later.
Smiles,
Grinity