My 4 month old will not take baby rice or rusk without sweetner. Can you advise me what sweetner I can give him.
This is not uncommon; if you mix the cereal with formula, you can add a bit of pasteurized juice instead of water, or, as suggested above by @Layna, add a bit of either fruit puree or a sweet vegetable (carrots, squash, sweet potato, etc.) to the mix.
You should avoid artificial sweeteners and honey for health reasons, and most doctors would disapprove of adding sugar or other concentrated sweet; the fruit/veggie is a much better option nutritionally.