If out door I would put joggers on the bottom, and sweatshiret on the top. Most kids will be wearing thier cherished Man U (or similar) kit underneeth, but don't worry about that. It's been quite warm today, but if it's cold, hat and gloves. He can always take them off when he gets hot.
Football sessions for kids - girls and boys - that age are just good fun, they just play 'running around' games with a ball, there's no pressure at all in it. It shows them how to listen even if there are lots of distractions, which is excellent thing for little ones.
Mine has been going for months without a foodball kit, just normal trainers and normal shorts/joggers. He was in shorts yesterday in the rain and he loved getting all muddy. He had mud in his ears!