Cuddly soft toys are most certainly age appropriate, and in fact most of the Pokémon plush toys are at least age 4+ if not higher. They're great for creative play, and are especially useful if there's nobody else to play with (as the plush can take the role of the other person - think Calvin and Hobbes). This kind of play is extremely valuable at this age (and continues to be valuable as they get older) as it lets them explore different things from their life and grow emotionally in a safe setting. It also helps develop empathy and social skills.
Books that are aimed at a younger child will not challenge him, but it's reasonable to separate "reading for fun" from "reading challenging books". My children, who are now pre-teens, still read picture books from time to time; they're both extremely high achieving readers (3+ grade levels ahead), so it hasn't stunted them meaningfully. Instead, it's a fun thing for them to do. They also read more challenging books, but that's separate from reading fun things that are nostalgic or just fun. They also may pick up more complicated concepts than they did as younger children - think about how rereading a book as an adult works; this won't apply to every picture book, but some are quite complex and have value for both younger and older children.