There are many ways. But I do it in a tricky way! So, this is my opinion.
The trick is to include 'WE' rather than 'YOU'. That said, you are talking about him but not in particular. When you call yourself 'we', you take the entire company but then when it comes to take decisions, that 'we' actually means 'him', the manager! That's what I said, there is no harm in using this trick!
See the difference it makes...
I suggest you that you recruit more design engineers and focus on designing of the new product line.
I think we should recruit more design engineers and focus on designing of the new product line.
[I just replaced suggest with think and you with we].
That way, you indirectly advise him by giving your humble suggestion. You put your point without being dominant. It's polite, indirect, easy and tricky! :)