Which JR mattress for ottoman storage bed

Hello there, would appreciate some advise before committing to purchasing a mattress please.I wish to replace my sons mattress ( small double) on his lift up storage bed (lifts from foot of bed by strap)My son is 20yrs old and is very much of average weight.The base has sprung slats which on information from your site we intend to cover with MDF once new mattress is purchased.I would also be grateful for direction in which topper is suitable for the new mattress.Thankyou for your time.JO