I was always lax with this, so understand where you are coming from.
DD is now 8.5 and is a grazer. She isn't fussed about big meals.
At a certain age I enforced table manners, not sure if it was as early as 20 months (may have been) so that if she wanted to get down from the table she had to ask (which may be a problem with your DS speech delay), say her 'please' and 'thankyous' etc
I think that "eating properly" depends on the child and what fills them up, not what is considered socially correct at such an early age.
What's important is that they eat. If DS is getting down from the table and then 10 minutes later asks for more food, it is worth while persevering and making him eat more of his meal.
I am sure I will read other comments blasting my theory out of the water. This is what I did for my child based on my circumstances, to suit our needs.
As with you OP, if it suits you and your routine and he is eating well, I wouldn't be fussed.
However, do not forget that at some point in the future, it will be important that he sits and waits and eats his meals in the correct order - think about going to school/family meals out.