I would recommend your son waits before doing anything at all to the garden other than cutting grass, etc (though if you want to post photos, they might be useful). The standard advice is to wait a year when you move, if the garden isn't a complete wreck, because you don't know what spring/early summer bulbs or other plants might be present that you yet can't see. Once you see what happens over a growing year, next autumn would be the perfect time to put in action any changes or new planting, including planting bulbs.

For instant colour now, you could buy some late flowering perennials (asters, Sedum Autumn Joy, Rudbeckia and so on) and pot them up, or just get some pansies and put those in containers to tart things up a bit with colour.