Count values of a column
This will solve your problem, use SUM and join in 1 query.

SELECT SUM(`kilo`) total_kilo
FROM `person` 
    INNER JOIN `products` 
    on `person`.`product` = `products`.`id`;


Or selecting/counting for user 1 alone

SELECT SUM(`kilo`) total_kilo 
FROM `person` 
    INNER JOIN `products` 
    on `person`.`product` = `products`.`id` 
WHERE `person`.`user` =1;


Or selecting/counting for every user

SELECT `person`.`user`, SUM(`kilo`) total_kilo 
FROM `person` 
    INNER JOIN `products` 
    on `person`.`product` = `products`.`id` 
GROUP BY `person`.`user`;